It’s gonna take me all day to read the posts in this thread, so I wanted to make my lone observation before jumping into that:
As much as it annoys me for Oda to introduce a bunch of characters each arc that ultimately have less agency in their own lives/conflicts than Luffy and his comrades, he usually makes each one of them unique enough to be independently intriguing. Aside from the obvious ones (like Kuina/Tashigi), he seems to largely avoid recycling character designs. This would be impressive enough if there were only 50 characters in the series, but he’s given us, fuck, 500? More?
Nearly from the beginning, with Buggy’s circus crew and Gaimon in the box, to the various exotic races in the world (giants, minks, fishmen/mermen, sky people), he takes these concepts and completely makes them his own - I just finished reading Skypiea again, and found myself paying more attention to the random denizens as they scrambled from Enel’s wrath, just noticing the thought he put into each character that shows up for one panel and never again. Better posts than mine have been written about his ludicrous attention to detail, so this is nothing new, but it still blows me away more each time.
Character design is kinda big for me if I’m gonna stick with a series. Bleach was one of my favorites for that exact reason, whereas half the characters in Naruto are practically identical to one another. For One Piece to always make each arc feel like its own story thanks to the people always resembling one another without looking too much like folks on other islands, it’s kept things fresh without sacrificing much in terms of plot. Glad someone created this topic here, because I’ve never really expressed these thoughts before.
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I’ll also say this - Kaido would look much cooler as a red dragon (like that picture you posted
@Topi Jerami), and King is my favorite of the Calamities by design.
Here are the characters I can think of (after just waking up) whose designs I enjoy the most, in no particular order:
Shanks
Marco
Zoro
Jinbe
Franky
Brook
Vivi (I’m realizing I just like distinctly colored hair)
¯\_(ツ)_/¯
Caesar
Pedro
Perospero
Amande
All the Vinsmokes
Urouge

Inazuma
Kidd
Hawkins
Bonney
There are probably more, but y’all don’t need a refresher on half the characters in this series. Having this many examples just illustrates my point above.