Luffy: Stayed the same for the most part, though 2003 onward Oda got more confident with his design and started to look a lot less like Shanks.
Zoro: Got way better and keeps on getting better.
Nami: 2003-2009 was her best design period, and she had way more expressions then too. 2010 onward she's kind of a generic super model.
Usopp: Vast improvement, especially on the perspective of his nose and hair. Looked best in Water 7/Enies Lobby to me.
Sanji: Sort of going back and forth these days, looked his best during Water 7.
Chopper: Looked pretty good in general before the timeskip, he was cute but not too cutesy.
Robin: Like others have said her design suffered the worst and looks like timeskip Nami with slightly smaller eyes. I think she looked really good on Sabondy.
Franky: I've gotten used to his timeskip design but I enjoy Water 7 Franky a lot more.
Brook: Has mostly stayed the same.
Oda's art style sure has changed a lot through the years,
from it's Kenshin mixed with Dragon Ball period:
! ![](http://img.batoto.net/comics/2011/09/21/o/read4e79b7047f040/Chapter 3 - Page 3.png)
to a more Disney looking aesthetic with Baratie and Arlong
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the very Tex Avery looking Little Garden and Alabasta
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to starting to find it's more original style with Jaya and Skypiea, and getting better backgrounds
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it got more angular in Water 7 and Enies Lobby, starting to solidify Oda's style
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to the visually arresting Thriller Bark
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and so on….
Right now it looks okay but the panels are way to crowded with detail.
I don't think his women look bad either, background female characters all look different from each other. The important ones though all have a samey body type though.