Hello! We're back with best in class! Honestly I had been intending to do assassins second and was going to release this a while ago buuuuut I figured it might be best to wait for Kama to hit NA. Now before we get into it here's a refresher on the rules:
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.

1)AOE assassin contestants:
1) Shuten-Douji (5-star arts assassin): shuten was surprisingly the second AOE assassin to ever be added into the game (with the first one being phantom but...... I mean it's phantom, there's no saving him). By now I think we all know what shuten does, decent gains on cards and an np which doesn't hit all that hard but applies a laundry list of debuffs. She has obtained an np strengthening which added toxic to her debuff list..... which doesn't help at all seeing as how most poison debuffs don't stack so they don't work like burns or curses in that sense. She didn't get a battery until recently in jp so she's been missing that for a while, solid all around but she doesn't truly benefit from castoria and even tamano can't really push her damage to the super high values needed for some of the more high end boss fights.
2) Cleopatra (5-star buster assassin): Cleo for the longest time was the only buster AOE assassin and, oddly enough, both her and the other buster AOE assassin in NA (yu mei-ren who will not be on this list) fill very similar roles. Cleo's main goal is to loop her np as much as possible and generally hit like a freight train when her no goes off. She can increase her np gain as well as give her self passive np gain plus she has hard survivability (although it's tied to her c star battery so that kind of sucks). Generally a solid servant even if she isn't exactly consistent, her best support is probably waver since she's more of a farmer then a boss killer but you can run her with double merlin + plugsuit abigail if you want to pump her damage through the roof and keep her passive np gain up.
3) ushiwakamaru (4-star quick assassin): putting ushiwakamaru on this list feels disingenuous because there is a better assassin with the same role as her but that assassin is jp only and there's still a very real chance that ushiwakamaru is better then the other NA assassins. She can loop and charge her np with animal communication and she's incredibly good at generating stars. She's technically capable of skadi looping but it's hard and there are better choices.
4) Gray (4-star buster assassin): gray hits like a truck and that's about it. Her star generation is remarkably bad for having a triple quick deck as an assassin and her np generation isn't high enough for her to comfortably loop with her one singular arts card even in Card lock set-ups. If there's one thing she is good at though it's damage. She's got the highest damage out of the buster assassins but her inability to farm makes her a bit of a hard servant to seriously consider for best in class.
5)kiichi Hogen (4-star quick assassin): she's ushi assassin but better and basically tailor made for skadi looping. All in all she's like the perfect AOE quick assassin and she's best about everything you could possibly want but her main problem so that outside of skadi setups she becomes a lot less effective as her kit is so tailor made for skadi that it falls flat on its face when having to work with other supports. Her second skills can be used as team support if you want to use her in like a set up where she produces a metric ton of stars and then feeds them to another dps but they kind of set up usually isn't very effective when stuff like 2030 is just out there existing.

2) single target contestants:
1) Jack the Ripper (5-star quick assassin): jack is a servant who I'm hard pressed to even put on this list because we've had a lot of assassins come out since who have better skills and nps and stats and are just better in general. Her kit is pretty standard, she has an evade tied to her quick buff which is problematic but it usually works out thanks to her play style. Her second skill is a buff removal (almost as good as buff block but not quite) and her third skills is a.... targetable heal which isn't all that great to be honest but hey it can help in a pinch I guess. Now, what really makes jack great is her ability to just create a pile of stars when you so much as glaze her finger over just one of her quick cards. Her np is also pretty amazing for a servant who was released so early on, it's basically a single target nuke they completely destroys female opponents (it should be noted that Jack's np actually applies special attack damage against females to ALL her attacks that are made on that turn). I think when talking about best in class, we have to talk about Jack because at the end of the day she still does her job better than any other servant even if that job isn't exactly useful.
2) Ryougi Shiki (4-star arts assassin): ryougi is another servant who, much like Jack, has rather simple skills and np effects but what makes her useful is that she hits like a truck and can pretty easily loop her np with castoria or even just tamacelsus supports. Great damage dealer and boss killer even if she is rather straightforward.
3) first Hassan (5-star buster assassin): Hassan has a lot of skills and a lot of effects which all in the end add up to dealing a whole bunch of damage 1 time. King Hassan is great for old school style bosses but like most buster servants, he just hasn't managed to keep up with the while break bad thing so his damage and viability has suffered. He is the best buster assassin by far but I'm just not sure if he's better then his quick or sets alternatives.
4) li shuwen (5-star arts assassin): the arts assassin who's np gain is so ridiculous that he can loop with a one hit arts np. He's basically just a more Crit oriented version of shiki and while they're both very good, I think Shiki does beat him out because of her higher np damage. Depending on the fight and the set up, li will perform much better.
5) Kama (5-star quick assassin): now the moment you've all been waiting for: Kama. Kama is an extremely solid servant, point blank period. Even if Kama wasn't an assassin she would still be up for best in class for a couple key reasons, she's the only servant capable of complete class advantage against alter egos, she's the only servant with a viable way to boost a servants overcharge (until himiko), she's got a guts that doesn't have a time limit so she can basically just stay alive forever in the right set ups AND she can skadi loop with ease thanks to high hit counts and her three turn quick card buff on her np which is further boosted by her overcharge buff on her S1. It's also worth noting that if you use her S3 into her np then you get what is essentially a guaranteed charm against anything that doesn't have either mental debuff immunity or charm immunity (which is a lot more common on male enemies this day because of the snake sisters but I digress).

3) Support contestants
1) fuuma Kotarou (3-star quick assassin): support assassins are hard to come by, the class itself is inherently support focused as they're meant to produce c-stars but they're np and skills just usually aren't built that way. Fuuma is one of the few assassins who does lean support thanks to having quite a few debuffs attached to his kit, the other is Mata Hari and she's pretty hit or miss (literally). His basic skill set allows him to decrease attack (and crit chance) and apply a targetable one then evade (a skill which is incredibly useful and should not be looked down upon). He can also decrease debuff resistance for the enemy (which is basically useless) but after his rank up he can decrease debuff resistance and defense of all enemies while also applying a 1 time buff block. His np also has confusion on it which means there's a possibility you could skill seal your opponent on one random turn within the five turn window. Skill seal is, I'm gonna be honest, pretty bad. I've mentioned that buff block and buff removal are pretty great effects in the past because they essentially cause your opponent to waste actions. If your enemy spends two off their actions buffing with buff block on them then they only get one attack, skill seal doesn't work like that. Skill seal just means your enemy gets three attacks which gives them more chances to just straight up kill you.
2) wu zetian (4-star quick assassin): wu is a support who just gets better as the game moves forward. Her basic support capabilities were as a quick support thanks to her empress charisma (which buffs both attacks Dan quick performance) and torture technique, outside of that she was basically just a poster girl ot plug suit servant. Since her original release she has had TWO rank ups which make her incredibly useful outside of poster girl, the first is her S1 strengthening which, after the upgrade, allows her to decrease one enemies defense, place a 1-time buff block on them, and tack a weak 1000 damage burn onto them as well. This partnered with her np strengthening, which gives her the ability to remove enemy buffs on her np, makes her a great servant for buff heavy fights where you need a more budget options to deal with the buffs coming in (not as good as servants like murasaki but also not as bad as say... Mephistopheles).

And that's all the contestants. There weren't a lot of support assassins to choose from really (4 total) and I didn't think emiya assassin or Mata Hari were wort talking about so they didn't make the cut of you were wondering about those two.