Unless I'm forgetting someone, currently the show has ten animation directors going by who's been involved over the past year or so:
Keiichi Ichikawa (also chief animation director)
Yusuke Isochi
Kazuya Hisada (also character designer)
Isamu Takara
Shuuichi Ito
Midori Matsuda
Kenji Yokoyama
Shigefumi Shingaki
Masayuki Takagi
Toshio Deguchi
Of those ten, I'd rank Ichikawa, Ito, and Takara as the best overall with Isochi and Hisada coming in right behind them. Most everyone else is solidly "decent" in my eyes except for Deguchi, who is far and away the worst of the group, and Shingaki, who's taken a big dive recently. For some more detailed thoughts:
! Keiichi Ichikawa: Before joining the show he served as animation director and character designer for the Episode of Sabo and Heart of Gold specials, and he was also the character designer for Saint Seiya Omega a few years back. I find his One Piece style to be influenced by Masayuki Sato's and Yoshihiko Umakoshi's work on the recent films (Umakoshi also shared the role of character designer on Saint Seiya Omega), which is most apparent to me in the thick, bold lines he uses.
Kazuya Hisada: As character designer he's responsible for the reference sheets the rest of the production uses and the overall look of the show. During his tenure in that position starting from Sabaody he's modernized the look of the show to be much closer to Oda's current artwork after Noboru Koizumi's kind of lanky, angular look that originated from the East Blue/early Grand Line days. Hisada's been around since the very beginning, and if you know what to look for his art's got kind of a boxy, straight quality to it on the show itself, not so much in the reference material; most noticeable when he draws characters in profile. He's had more "sakuga" bits of key animation in the show lately compared to before.
Kenji Yokoyama: Yokoyama has also been around from the start - I think episode 3 was his first one. For a long time I thought his art was pretty simplistic and wooden, but he's improved a lot since Hisada became the character designer and now I find his art to generally be very crisp and on-model, plus he seems to be particularly skilled at drawing the more complex, crazy designs. The trade off however, is that his episodes rarely have complex animation unless he gets a lot of help or is paired with a really good director, and utilize a lot of noticeable shortcuts.
Masayuki Takagi: The last of the "Old Trio" who've been AD-ing since East Blue. Since Katsumi Ishizuka stepped away his style's the closest to Koizumi's, but it's kind of stretched horizontally and otherwise exaggerated. When his episodes look good, they look good. When they're not, they tend to look kind of sloppy.
Yusuke Isochi: Joined in Fishman Island. His style's essentially "Yukiko Nakatani Lite" since they're actually part of the same studio if I remember right.
Shuuichi Ito: Ito became an animation director towards the final third of Dressrosa I believe, but he's been providing key animation for a quite a while prior, even as far back as Movie 1. I'm still trying to get a solid handle on the quirks of his art, but recently it seems like he's adopted Ichikawa's method of bold shading lines.
Isamu Takara: Became an animation director towards the end of Dressrosa, and has been providing key animation since at least Punk Hazard. His animation style is kind of loose and smeary like you see in a lot of anime these days, and that reflects on his art.
Midori Matsuda: Joined in Zo, but contributed key animation on a fairly regular basis during Dressrosa. Definitely takes after Hisada in her art style.
Shigefumi Shingaki: Joined in Sabaody. His art kind of had a sharp look to it at first, then gradually became kind of "thick" as well. Unfortunately his episodes had a big drop-off in quality after the timeskip, but he's shown some signs of recovery lately.
Toshio Deguchi: Joined in Amazon Lily. Every production has a weak link, and he is One Piece's. On the whole his art's pretty boxy and simplistic, and he'll probably be around for a good long while yet due to being the designated "garbage time" animation director of the bunch.
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