The Marines in general are good characters because one of Oda’s themes that he writes a lot about, and is interesting, is justice and what it means to people. The most interesting Marines are all the ones who question or have their justice challenged.
Like Kizaru was not especially interesting before Egghead.
Akainu so far is not the most interesting character, but there are hints to more to him. He was a ferocious villain in Marineford, which is actually good because there’s a place for that in any story, but I woulsn’t say so far that makes him the most enthralling character.
Kuzan, if Oda sticks the landing with him, might be the best written character in the series.
Fujitora is well written, but if he’s criticised it’s that he’s a tad too similar to other Marines, particularly Smoker. Who was the best of the bunch and it’s criminal what Oda has done (or really, hasn’t done) with him.
Ryokugyu is too early, I don’t like the Akainu fanboyism, the hope is that the CD thing is him being a pessimist realist rather than a weirdo worshipper.
Sengoku is okay. Nothing too special tbh.
The Warlords and the Yonko don’t quite have a common theme connecting them the way the Marines have justice, so it sometimes doesn‘t quite make sense to look at them all as a group. Like, connects Jinbei and Moria? Other than being Warlords, nothing.
But of them
Whitebeard: I think a generally strong character, but not the most interesting. Powerful, gruff but good old man who dies isn‘t exactly the most unique story ever told. Rocks may add more to this.
Shanks: I think Shanks is actually a pretty consistently written and good character, as much as people are now bitterly complaining that he’s actually playing a role now.
Blackbeard: connected to Kuzan for a reason, if Oda sticks the landing with him then it could be a tie between the two for best character.
Kaido: what a waste. Such a dreadful, dreadful fumble in the end from Oda. I’m not sure I’ll ever recover from those last few chapters of Wano.
Big Mom: again, a waste. Interesting case is that her character is quite well written and consistent, but ruined by Oda utilising her terribly, culminating in her being written off, from the looks of things, in an arc that had nothing to do with her.
Warlords:
Mihawk: it is time for him to play an actual role, but he’s managed to pull off the cool as fuck, haughty bored aristocrat badass vibe very well so far.
Crocodile: Just a great villain. If Oda wants to give him more background and stuff, fine, but honestly he’s great as is.
Moria: decent enough, but I‘m not sure there’s much of a role for him left? Would it have been better for him to have been killed by Doffy, going out as a warning for what happens when the will is broken?
Hancock: Tbh, not a character I spend much time thinking about. Oda doesn’t write romance especially well, and her gag about loving Luffy, eh, can be funny in small doses. Interesting backstory though, if Oda wants to develop that instead of just being in love with Luffy.
Jinbei: My man.
Doffy: I don’t quite like him as much as Croc but he was a pretty good villain. I do love his podcasts from Impel Down though.
Kuma: I think really well written. The whole idea about him being a target who took in everyone’s pain for years, how that ties with the great Zoro scene, his involvement in some real iconic One Piece scenes in general, big fan of Kuma.
Weevil: I‘m pretty sure Oda’s had to drop a Weevil plotpoint and doesn’t quite know what to do with him. He may end up regretting that he made the last Warlord a new character instead of like… Urouge or something. But who knows, maybe there is more to come.
Law: I think Oda is in a hard place with Law. His relationship with Doflamingo was great. But since that is over, not convinced that Oda has found much of an interesting place for him.
Buggy: as long as Buggy never becomes actually powerful I will always be a fan of his schtick.