Alright, we're back with best in class and this time we're wrapping up the casters. Now for a quick refresher on how this is gonna go down: I'll be determining who is the actual best in class in both NA and JP based off how well they do by themselves and in teams.

Best in class AoE (NA): I'm not gonna lie, this was a hard decision to make. All of the AoE casters were good for different reasons, sieg has low damage but good looping, Murasaki and Anastasia have good damage of utility but fall short on looping potential without high grade setups, Nito rise can basically only be used for farming but she's great at it, and Leonardo hits like a truck for the one turn she can use her np and then proceeds to wind up for the hopeful second np she'll get before dying... and then there's Nero.
Best in class: Nero Claudius (caster) (5-star buster AoE caster): so this may be a bit of a controversial pick but when I was deciding best in class I had to look at a servants entire kit: what are they bringing to the table, how does the kit interact with their ideal supports and teams, how does the kit allow them to function outside of that team, etc. and out of all of the casters Nero has the best kit out of all the casters in NA currently. Her s1 is a 50% battery and since she's a buster servant that is a very nice thing to have, her s2 buffs her attack and defense while also negating her class disadvantage against riders (although it's only defensive so she's still not really viable against riders) but what really gave her the leg up was her s3 which is a guts with an attack up which is targetable. Now I have Nero caster and I have never once not used this skill on her but the fact that it gives her survivability that she can use herself without relying on merlin or David goes a long way for her usefulness. On top of that she also has better damage than da Vinci with a non-zero chance of looping her np within that window in her ideal team (with double merlin you can use her s1 and both merlin's s1 to get her up to 90% charge which means if you used an arts card on the previous np turn or even just have both merlin np's effects up then you've got another np). Her np also ignores invincibility for any pesky challenge quests or bosses that like to use evade or invincibility skills (*cough* okada gilfest boss *cough*). I think what really sets Nero about the others though is that she's a buster servant and the other casters are all arts which means in NA their best supports are tamano and Paracelsus who don't always deliver on what they need (damage pushing). Merlin is an expensive support but Nero also has other options that are also extremely good (Shakespeare still allows for the looping mention in the double merlin aside and he can be more easily used with poster girl, Bunyan can also help push damage and is a great poster girl target), she's pretty much always going to meet the damage threshold and with proper skill usage, she can do that two turns in a row which just puts her slightly above the other casters.
Second place: Murasaki Shikibu (5-star arts caster): the long and short of it is that she has great looping potential even with just tamano and Paracelsus and nullify buff is beyond useful in boss fights/challenge quests. Not gonna go too in depth into it but yeah, definitely a close second.
Third place: sieg (4-star arts caster): one of the best caster farmers in the game even in NA where he only has Paracelsus and tamano. Unfortunately his damage is a little low for anything outside of that and he has NO survivability option even as far as stalling out turns goes.

Best in class AoE (JP); Alright so luckily there were no servants who were in JP and not NA for this category so we're working with the same general cast but the landscape has definitely changed. Immediately Nero is no longer in the running because castoria exists and there is just no way for merlin to push past that hurtle, which leaves us with the five arts casters. Leonardo and Nito are pretty incapable of using castoria (albeit for different reasons) to her greatest potential and I actually think they have different ideal supports (tamano for da Vinci and waver for Nito) so they're both out. So the final three were sieg, Anastasia and-
Best in class: Murasaki shikubu (5-star arts caster): I'm gonna be really honest when I say that it was a though choice between the top two. They both have great looping ability and both Anastasia and muraski have really good damage, what really set murasaki above Anastasia was the fact that her utility options (buff block>skill seal, np seal>stun((its super unreliable otherwise stun would obviously be better))) are just a lot better than Anastasia's. Muraskai can push up her damage with her s1 and s2 (albeit her s2 is best saved for later turns) and her s3 is pretty a decent team support skill that doesn't take away from her offensive capabilities. She's incredibly useful for both farming and boss fights (especially with buff block which, and I can't stress this enough, is amazing) castoria gives her consistent looping especially with black grail (in fact along with Anastasia, sieg, and schenzarde ((who wasn't considered for best in class because outside of farming it like a king Hassan boss fight she's completely useless)) she's one of the only casters capable of that) while also pushing her damage up through the roof. Great servant with incredible support options, really that's all there is to it
Second place: Anastasia (5-star arts caster): really the main thing about Anastasia is that she has higher damage then murasaki but only for one turn and then she's back to being below murasaki's damage. Skill seal and stun are both good but not as good as murasaki's options. Anastasia does have ignore invincible though so against bosses who have unavoidable invincibilities and evades she can still be really useful.
Third place: sieg (4-star arts caster); the world shifts around sieg but his damage is still too low and he still has 0 utility and 0 survivability. Great for farming but there are better options for boss fights or challenge quests.

Best in class single target (NA): single target casters were a bit harder then AoE because they are just functionally different. Single target servants in general have to be really good in challenge quests and boss fights because they're inherently not farmers so these servants had that much more to fight for. For instance there can't be a servant like sieg here who doesn't have the damage to perform super well in boss fights because they don't have farming to fall back on like he does. Medea is honestly a hard sell for best in class due to the fact that her damage is lacking and Sheba faces the same problems. Ilya has the second highest damage with double merlin but after one turn she starts to fall short and if you use her weird supports she fall behind in the damage race. Circe also has a lot going for her but she only really works as a semi-support and not a main damage dealer so she's out of the running too. That leaves us with shuten caster and-
Best in class: Xuanzang sanzang (5-star buster caster): once again the buster casters beat out the arts casters in NA because merlin is just better then tamano unfortunately. Xuansang has many of the same qualities as Nero (caster), she's a buster servant who's technically capable of looping but instead of needing two merlins she only needs one. Her s1 is an 80% charge and a 20% np damage up so it can either be used to get that first np out and have it boosted by a stronger ce like black grail or you can use it for a second np in a row. Her s2 is a taunt with damage cut which helps her build np charge and her s3 helps as well because it increases np gain (and star gen which is always nice since her np hits like 12 times, it also has debuff immunity so I guess she works as an Ilya support). Xuanzang is one of the only buster servants capable of looping her np 3-turns in a row with extremely good card draw and crits but her damage isn't exactly the best. While Xuanzang is definitely the best in class there are still servants like meltryillis and mecha Eli-Chan who hit like trucks and have an easier time doing it (and as a bonus mecha Eli-Chan is a welfare it should also be noted that Nero faces similar problems except with kiara and king protea but neither of them are welfare so it's less pronounced).
Second place: shuten douji (caster) (4-star buster caster): as much as I don't like shuten, it hard for me to deny that, as a welfare, she hits like a truck. In fact she has the highest damage of all the single target casters but that's only on her one burst turn and outside of that turn her damage isn't as good (but it's still better then the others). She can't really loop for three turns but she can safely loop for two with crits and dreamlike charisma (although that further gimps her damage) and crits are easy for her to achieve on her burst turn. She doesn't have hard survivability but neither does Xuanzang so it doesn't really matter.
Third place: Medea (3-star arts caster): on demand buff removal.... what you already know that Medea is good. Do you really need me to tell you that she's better than unreliable Ilya, low damage and no Utility sheba, and "I struggle to np more than once" Circe.

Best in class single target (JP): so you may be thinking that this is gonna be a repeat of the AoE section. The arts casters will overtake the buster casters because castoria exists... you'd be partially right. Medea still isn't in the running for best in class because her damage still sucks but the Queen of Sheba is a different story.
Best in class: Queen of Sheba (4-star arts caster): the Queen of Sheba hits like a truck under the guidance of castoria and she has the potential to do it for three turns.... no really that's it. She hits harder then Xuanzang and as an arts caster she has an easier time looping her np due to her being an arts servant. The only thing she's really missing is hard survivability. Having said all that I still think that she falls short outside of her ideal team but all the others have the same problem so I really don't know what you want me to do about that. Just like with Xuanzang however she is still outdone by alter egos except she's outdone but meltryillis, mecha Eli-Chan, and Sitonai now.
Second place: Xuanzang Sanzang (5-star buster caster): guess what, Medea still hits like a wet noodle compared to the others even with castoria and not that much else has changed about the landscape of JP. Xuanzang is still a good damage dealer so if you have her and don't have a better alter ego then she's still really good
Third place: Medea (3-star arts caster): she still doesn't hit hard but she still has buff removal on-demand which makes her great for buff heavy fights, she doesn't really work with castoria and I'd even go as far as to say she prefers tamano but she's probably best used as a one time buff removal with poster girl. Shuten doesn't take her spot because Medea at the very least does hit a lot harder then she does in NA with castoria had she now has even better looping capabilities so there's that.

Best in class support (NA): this is by far the easiest section to do despite the fact that it has the most entries. I want to just say that all the supports mentioned in the part 1, even if they were just honorable mentions, are really good and chen gong and Paracelsus are also really good even it they didn't make the list. Casters are a great support class but at the end of the day it really comes down to servants like merlin, skadi, tamano, and castoria for best in class.
Best in class: merlin (5-star arts caster): ok so I know this is a controversial pick because skadi is really good but skadi is really only good in top of the line expensive set ups while merlin is technically usable even if you don't have any non-event 5-star craft essences and for me that puts him just above skadi. He's basically everything a buster servant could want and he also puts in good work on arts crit teams (in my opinion he's actually a better partner for BB (4-star) then tamano and the same can be said about nursery rhyme). I mean we should all be aware of just how to good merlin is at this point.
Second place: Skadi (5-star arts caster): she's definitely a servant who works better if you have the top line servants and craft essences that she wants to work with but outside of that she's still pretty good, if a bit difficult at times. One skadi actually isn't even that broken, it's when you combine two with an already good servant (meltryillis, parvati, Achilles, etc.) that she breaks the game in the way we all know and love (or hate).
Third place: tamano-no-Mae (5-star arts caster): honestly it was hard to decide between her and waver. Waver definitely has a place on most teams while tamano is limited to arts and only really seems to excel on arts stall or with servants who have really bad downtime. Tamano wins out for me just because she makes servants who aren't viable more useful, for example you can actually use her with Edison to make him half decent and she already good servants like mash through the roof but the problem is that these are both supports and most dps servants don't have this problem, especially if they're Arts based. Tamano helps solve a lot of problems for arts servants but she also doesn't push damage that much and she isn't fixing the problems of the top tier arts servants because of that, still really good but definitely not ideal.

Best in class support (JP): let's just cut to the chase.
Best in class: Artoria pendragon (caster) (5-star arts caster): a support capable of pushing even mid-tier arts servants to heights that skadi struggles to push top of the line quick servants. Castoria allows for easier farming, easier challenge quests, and just easier living and she does it all without requiring super expensive set ups (you can run double castoria with sieg with any CE grouping and still successfully three turn farm). While she definitely does better with top of the line arts servants (summer kiara goes brrrrr) she is capable of working on just about every arts team. She isn't as good as tamano for arts stall in my opinion but if you run her with merlin and himiko then you've got a buster based stall team that will never take damage ever under the right conditions while still dishing out crits. I mean there's really not a whole lot to say, she's the perfect arts support, or at the very least the most perfect one to have released so far.
Second place: skadi (5-star arts caster): so while skadi hasn't changed at all, the landscape around her has. There are a lot of cheaper servants who work well with her now so it's not just the top tier servants who benefit from her presence, she falls short of castoria's excellence but, with the double skadi setup becoming a little cheaper, she outperforms merlin in the current JP landscape in my opinion.
Third place: Merlin (5-star arts caster): merlin hasn't changed but the idea of a buster servant being anything more than a crit machine certainly has especially with quick and arts servants getting better and buster servants only really receiving three new 5 star servants in two years (Arjuna alter, who is more broken because of his own kit, Orion who is once again broken because of his kit, and himiko, who works pretty well with merlin). Still not bad by any means but, dare I say it, a little outdated.