So I'm going to start a little series I like to call best in class that essential will go over just that, who is the best in that class of servant. For a quick refresher there are 13 classes of servants, the main seven are saber, archer, lancer, caster, rider, assassin, and berserker, the additional six are: ruler, avenger, moon cancer, foreigner, alter ego, and shielder. With the exception of shielder, all of these will eventually have a best in class made about them. So with that out of the way, here are the rules for best in class.
1): there are three categories for best in class: AOE, Single target, and support
2): in the first part of each best in class, I will narrow down the contestants and in the second i will choose the actual best in class
3): servants in both JP and NA will be considered and best in classes will be determined for both regions.
4) all welfare and 3-star or lower servants will be assumed to be at np5 while all 4-star and above servants will be assumed to be at np1, I know certain servants can perform way better at high np levels but I have made the executive decision to not care.

And without further ado; may the games begin.

1) AOE caster contestants: There are surprisingly few Good AOE casters so this should be quick
1) Nitrocris (4-star arts caster): nitocris is a what most people would call a quick and dirty kind of servant. She wants one thing and one thing alone, to use her np as many times as possible and hit the instant kill on the strong enemies she can't kill. She's a great farming servant but I don't know if that will carry her to best of class
2) Leonardo da Vinci (5-star arts caster): np damage foes brrrrrr but np gain sucks. She's got plenty of ways to recycle her no for multiple uses including golden rule body and a np battery and her np hits pretty hard with some support from units like waver and tamano but on her own she's pretty lackluster so while she's definitely in the running, I'd say her chances of being best in the class are low
3) Nero Claudius (caster) (5-star buster caster): umu! Everyone's favorite Roman emperor takes the stage in her swimsuit version. She can't loop her np very well by nature of being a buster servant so she's not the best farmer unless you've got a waver hanging around your Chaldea but there really isn't a AOE caster who hits harder than her and the targetable guts in her s3 gives her some amount of team support. Nero is definitely in the running for best in class and her chances of winning are pretty good.
4) Anastasia (5-star arts caster): finally a caster capable of easily looping their np... but they still doesn't make her all that. Her buffs are mostly one turn so her loop capability is a lot more burst than sustain which means she not as useful as servants like nitocris who have an easier time looping. When it comes to damage, she hits harder than most of the other arts casters and she provides more support with the combination of attack down, defense down, skill seal, and stun.
5) sieg (4-star arts welfare caster): sieg is the first welfare servant to make it to the candidates list and not just because there aren't any other welfare AOE casters. Sieg is one of the best servants in the game when it comes to arts looping and his potential only grows with castoria looming in the background of grand order NA's future. His damage can be a little low and his anti-dragon niche is almost completely useless (I mean there are sometimes dragon assassin enemies I guess), but at the end of the day he's a great servant for farming. Farming is what most of grand order is but it's not everything and his team support and challenge quest usefulness are on the low side so his loop capability and castoria compatibility don't necessarily guarantee him the win here.
6) Murasaki Shikibu (5-star arts caster): our last aoe caster contestant is actually the last AOE caster to be added to both NA and JP (Chen gong is more support than AOE) and she's not half bad. She's got decent loop capabilities even in NA where her best options are Paracelsus and tamano and I'm sure she'll only get better with castoria. Buff nullification is an amazingly useful trait on a servant and np seal is equally useful. On top of all that she also supports the team with damage cut, debuff immunity, and buff removal resistance for her whole team. The main thing she's missing is some kind of arts buff but with the abundance of arts support she's just fine. What lets her down the most is her damage which is still good but not as good as some of her fellow contestants.

2) single target casters: so while there were six contestants for best AOE there are also 6for best single target so this list will be about the same in length.
1) Medea (3-star arts caster): the og caster and the very first caster to ever be added in to the game. On first class she seems like she should have aged and then she laughs in the face of that assumption and smacks her enemies in the face with an unlimited stream of nps. She has absurdly high np generation and an absurdly high np refund effect, on top of this she's got rapid words of divine which is a pretty powerful charge skill She can increase her np gen and cleanse her debuffs with Circe' as teachings and argon coin.... well best not to talk about it. Oh and her np also removed buffs from whoever she hits. Now while she may seem like the obvious best in class, her damage sucks, she needs support to actually do anything and she needs good cards to loop successfully. When she works well she cleans house but when she doesn't she becomes a brick which can drag the team down.
2) xuanzang sanzang (5-star buster caster): ora! Ora! Ora! Xuanzang busts onto the scene with incredibly damage and team support and wonderful consistency. The problem with her is pretty simple, she's consistent but not consistently amazing. She's caught between being a support and being a damage dealer so her damage isn't stellar and her deck doesn't allow her to buster brave chains. While Medea can work wonders in the right situation, xuanzang swaps a high ceiling out for a higher floor.
3) Ilya (5-star buster caster): honestly the thought of Ilya winning best in class both makes me happy and want to laugh because it's incredibly unlikely. Ilya hits harder than any other single target caster and she's got this nifty little np gen and star gen buff on her np the problem comes in the fact that she reduces her own attack and defense after her np goes off and she needs niche supports to help her out. Kuro and Miyu can both help her out and BB can help her out a lot too but even those are just bandaid solutions to the bigger problem. She hits super hard the first time and then every hit after that is just doing less and less damage if your using the supports that any other servant would normally use (in her case merlin/waved) and if your using the supports that mitigate this problem, her damage is still lower overall.
4) Circe (4-star buster caster): Circe, or caster of okeanos, is an incredibly weird servant. There's a trend amongst buster casters of them having AAABQ decks (with the exception of Nero) and Circe falls into that and she's one of the few servants who seriously benefits from that. Her np gain is actual trash she has to supplement than with arts chains so for once her deck isn't what's holding her back. She's got the rapid words of divine skill of instant np and she's got a defense down to help push her damage. After hearing all this you may think she's a really good servant, and she is, but she's not really all the. Circe really can't "loop" in the way we think of looping and while her np has an extremely powerful effect on it (it's a guaranteed stun against just about any servant), it doesn't do very much damage. She's got a lot of team support to her but she is a single target caster and dealing damage and disrupting the enemy does seem to be her game plan so I struggle to call her a support. Her chances of best in class are low but her chances at best in Robin Hood support are higher than you might think.
5) Queen of Sheba (4-star arts caster): the Queen of Sheba or caster of Midrash is a servant that I think a lot of people aren't familiar with since she was released alongside abigail Williams who is considered the worst of the foreigners and there were no other 5-star units on that banner. She however is actually not that bad of a servant, she has a host of team wide buffs and a self crit up that goes up to 100% at max level and her np comes with a whole host of debuffs that neuter an opponents ability to both recover and come back from defeat. Sheba's main problem is that her np hits like a wet noddle and she doesn't really be right from arts support as much as you would want her to. I'm sure castoria fixes her damage problems and lets her loop with ease but that doesn't help her hit crits which is really what she wants to do and if you run her with double merlin to help her hit crits then her np doesn't do any damage. Sheba wants a lot of things that she can't really produce herself and that's what really lets her down.
6) shuten douji (caster) (4-star welfare buster caster): shuten douji is the only welfare single target caster and she's also kind of disappointing. Out of all 6 casters on the single target list four of them have AAABQ decks with a buster noble phantasm, if you're thinking to yourself "but don't only four of them have buster np's" then you are correct. All four buster single target casters can't buster brave chain their np and they can't arts chain their np for higher gains either so in the end they kind of lose out on the advantages that their deck should provide them. This is most notable on shuten. Shuten is a burst damage dealer (with a weird anti-demonic charisma skill thrown in instead of a second burst skill for... lore reasons I guess) so most of her damage is focused around her s2 which is a fairly standard arts and buster buff with a c star absorb, all of which last one turn. The problem is that this skill is on a 6 turn cool down so the up time on it is terrible. Her s3 is a charisma with an anti-demonic trait on it so she has a definite niche against enemies like the douji's. Her s1 is an np seal (1 turn) and hefty critical down (3 turn) on a 6 turn cool down so the up time is okay and it gives her some survivability but it's still not great. Her np poisons the enemy so if you wanted to run her alongside Robin Hood then I guess you could but I wouldn't recommend it. Overall shuten is a good crit burst servant and her np is incredibly strong when she using both her burst skill and charisma along with outside support but the uptime on her skills is horrible and for the 5 turns (minimum) where she isn't doing damage, she's more a liability than the other casters so far.

3) support casters: there are a lot of support casters.... like a LOT but I'm only including the best so hopefully this won't be too long.
1) Hans Christian Andersen (2-star arts caster): the og support and definitely one that EVERY MASTER should raise even if you have supports who are technically "better" supports. Hans is one of the best crit support servants in the game especially after the rank up which he just received in NA. His s1 is a crit buff which could use a bit higher percentage but its team wide and on a 5 turn cool down so I'm not complaining, his s2 is a large battery which helps him use his np more often which we'll get to later, and his s3 is a godlike version of innocent monster which is a TARGETABLE buff that generates 15 stars and charges 10% np gauge every turn for 3 turns on a 5 turn cool down at max level. His np is one of the chance based ones which means it's not super reliable but it has the potential to act as a charisma, a defense up, and a star gen increase all while healing the team. Hans is by in the large the best of the budget supports but he's not the only one who will be appearing on this list as I do have to look at the others.
2) Wolfgang Amadeus Mozart (1 star arts caster): where Hans works in every comp, our second budget support caster is definitely more at home in arts crit team. Mozart is a burst support so he's best used with poster girl since you really don't want him around for longer than it takes to use his s3 (which produces 50 c stars at max level) and s1 (which is a high percentage arts performance up for the whole party). He's not as versatile as Hans and he can become dead weight rather quickly due to the low uptime on his s3 and an s2 which really only works on teams that run other np power down options like Florence, Jeanne, or meltryillis.
3) Zhuge liang/waver (5-star arts caster): the de facto best general support who has managed to find a place in the tamano, merlin, skadi, and castoria metas. Waver's main draw is his ability to charge 50% gauge for one ally, and 20% for the other two, while also providing good buffs. His s2 is a defensive buff and his s1 and s2 he'll push damage on both normal cards and noble phantasms. How noble phantasm is also full of good effects, it decreases charge, lowers defense, has a chance to stun, and throws in curse for..... yeah curse is useless outside of Salome set ups but that's going to have to wait for the berserker best in class. What lets waver down is just how general he is, his buffs aren't as amazing as you might be led to believe by looking at their effects. On top of this he has the disadvantage of having really bad down time, his skills are on cool down for as long as their active and his card damage sucks, plus his AAABQ deck might mess up quick and buster chains outside of arts teams. Overall he's not a bad servant, far from it, but there are definitely better options.
4) tamano-no-Mae (5-star arts caster): Mikon! Tamano was the first of 4 support servants who were so strong that they changed the way the game was played. She has since become a bit too slow for most masters but there is no denying that when she released she was an incredibly powerful servant. Tamano's main draw is her np which reduces skill cooldown and charges np gauge (it also heals but that's slightly less important) and therefore reduces the downtime of both herself and the other servants on her team. She also comes with a massive three turn arts buff and a rare 3-turn np damage up for your main damage dealer. All in all tamano is a great support and she was the best support you could ever ask for when it came to early age arts teams and their stall tactics. She's definitely aged poorly as the game has gone on but she remains unmatched when it comes to supporting servants with bad down times.
Honorable mention: Medea Lily (4-star arts caster): I know Medea is often considered the worst of the healer servants and I agree with that but I think she beats out iri only because iri doesn't have debuff cleanse and that's half the reason I think masters want to bring healers in the first place. Healers are so niche that they're not typically good, Medea Lily is so niche that she becomes amazingly useful against certain bosses and with certain party members especially after she receives her last rank up (in NA) this year.
Honorable mention: Paracelsus (3-star arts caster); Paracelsus is not being considered because castoria does everything he does but better and also more. Paracelsus is great at making sure your arts units can spam their np as much as they want but his down time is horrendous and his one damage boosting skill isn't very large.
Honorable mention: Helena Blavatsky (4-star arts caster): Helena has absolutely no chance of even competing with top servants, in fact I'd argue that she's outpaced by the likes of Hans. Her buffs are weak, and her downtime is horrible. Her s1 however is extremely good and is a lot of masters only option for a 20% team battery since waver, merlin, and castoria are SSRs.
Honorable mention: Thomas Edison (4-star arts caster): a gimmicky servant with no chance at competing with others in his class. Edison comes packed with unique buffs on his skills like skill cooldown and overcharge but the downtime on these skills is so bad that a lot of the time the only thing they do is mess up the flow of battle.
5) Gilgamesh (caster) (4-star arts caster): Gilgamesh is the fourth arts crit support on this list so far and I struggle to say that he's a support as he is actually a pretty decent AOE damage caster however with three team wide buff skills, a defense up on his np and just the general role of being a star bomb who has the potential to take out side enemies or break a bar I think he leans more towards support. Where Hans is about general crit damage while providing some consistency and Mozart is pretty much a one time guaranteed crit turn, Gilgamesh is centered more towards consistent arts crits. Gilgamesh is probably going to be most useful on teams that use servants like kuro who have high arts hit counts and star weights since while under the effect of his skills they can actually produce stars on arts cards, it also helps that kuro has two quick cards for good measure. Gilgamesh is a great crit support but he loses some of his usefulness when paired with servant who have low star weights or against single enemies as he can't produce as many stars.
6) merlin (5-star arts caster): the first of the absolutely broken supports. The other entries are all good but merlin is the first one to truly break the game itself. Merlin is a buster support and as such his job is make your servants hit like a truck as often as possible. His s1 is a charisma with a 20% battery for the whole team which means if your using a non-mlb kaleidoscope then you can still use your noble phantasm on the first turn, or if your using a mlb tale of love and hope (or any of the other 60% charge CEs) and have two merlins or merlin + Helena, you can np on the first turn. His s2 is a full team one turn invincibility (so it's basically a better harp of healing and that skill is already really good) while also lowering the enemies crit chance and increasing your teams c-star drop weight. His s3 is the big money maker skill, it's a massive crit damage and buster buff that also increases the targets maximum hp. Pair that with Ishtar's s1 or any other servant who can boost their own crit damage and you can get some pretty crazy numbers. Merlin's main drawback is that the crit buff is only for one turn so if your planning on 3-turning a bods then you'll have to be creative with the way you do it (np-crit turn-crit turn is the best way but there aren't a lot of servants who meet the damage threshold of the first bar with one np and card draws and c star production can be a problem without 2030's and summer BB). His np heals the team for 1000 and charges the teams no gauge by 5% while also producing anywhere from 5-25 stars depending on overcharge and it does all of this for 5 turns, no that's not a typo.
7) scathach-skadi (5-star arts caster): skadi is the second of DW's broken support trio and (in my opinion) she's the weakest of the three. She can push damage and let's quick servants to loop their np but this can all only be accomplished under very specific (read: expensive) conditions. Her s1 is a quick card up and crit up on quick cards only, her s2 decreases defense and crit rate and her s3 is a 50% targetable np battery at level 10. Skadi's np has a laundry list of buffs but the important ones are a 1-hit evade for the whole party, and a crit damage up. Skadi struggles to actually charge her np so if you fail to defeat the boss by the time it's time for them to use their np then you'll find yourself in a situation where you've lost you're entire front row or at the very least one party member. When she's pushing damage, she pushes damage through the roof and beyond but her skills are down just as much as their up. At the end of the day skadi is a quick support and her ideal play style is just that, she wants to kill the enemy as fast as possible and if she fails then she's pretty much dead weight on your team.
8) Asclepius (3-star arts caster): we interrupt this chain of over powered supports to talk about the best healer in the game. There's really not a lot to talk about with this guy. He's basically a combination of Medea Lily and Iri and then some. His np provides guts and debuff immunity as well as hp regen. He can clear debuffs and charge np while increasing np gain and applying np seal. He's also got a massive Heal with debuff resist on it. His drawbacks are, you guessed it, massive downtime in which he does nothing but take up card space, the solution? Poster girl (or outrage after that comes out). Now I hear you saying "but doesn't poster girl solve the issues of every support who has massive downtime where they're useless?" And to that I say: yes it does but it usually works better for lower rarity units since they have an easier time dying (however higher rarity units can make better use of the attack up from poster girl). Overall Asclepius isn't going to be a servant whose super useful to people who can cut through waves after waves of enemies with their 3-turn teams, he is extremely useful for those of us who take 50 turns to best a cq (now for real I spent 50 turns bullying kiara with my level 64 king protea yesterday, it was very rewarding but also incredibly slow and there was more than one time where I really would have liked a good heal. I will mention that his guts can mess with poster girl but if your just using him for his skills then it's not that big of a deal, if you are planning on using his np then I suggest kaleidoscope with plug suit instead of poster girl.
9) artoria pendragon (caster) (5-star arts caster): I truly believe that someone at dw really likes arts servants and saw that they have been doing pretty bad since skadi released and then deciding to do absolutely everything in their power to make an arts support who was so broken then they would fundamentally change the way the game was played. The result: castoria. She's got a 30% team wide np battery that's tied to a charisma buff on a 6-turn cool down, a 20% targetable np battery with an np gain up attached to it and a 50% arts buff tied to an invincibility and a special attack up against threat to humanity (that last one isn't really important as there aren't that many threat to humanity enemies out there). Castoria essentially allows for mediocre arts servants to be some of the best in the game and allows amazing arts servants (like Murasaki) to out due some of the best quick and buster servants by a mile. As for her team wide survivability option, she's got her np with is a 30% team wide attack up (at np1 and personally I don't think it's worth getting supports past np1 if they're five stars) along with a 1-hit super-invincibility (real name is enforcement defense but it's basically super invincibility) that completely blocks damage even from enemies with ignore invincible. Her np gain isn't great until you consider that she buffs her own np gain with her s2 and she's also an arts support so you're constantly arts chaining and building np with her (unlike skadi). Her main problem is that she doesn't push damage as high as skadi or merlin but tamano can be also be used alongside her to both decrease the teams down time and increase the amount of damage pushing with her s1 and s3.

This was a really long chapter, second part won't be nearly as long but if you've made it all the way to the end then tell me who you think is the best caster in each category.

After publish note: you may have noticed that Chen gong is not on this list. Chen gong is a great caster but I didn't really feel like he belonged on this list, his role is a little weird in most teams and I don't think it quite qualifies as AoE or support. Personally I think Chen gong is an amazing servant but I just didn't know how to classify him and to be honest he wouldn't have won any of the categories.