Design talk, eh. Well, what the hell. Won't go too much in depth, but meh
Luffy: I always thought his character was blandly designed. Of all the characters in the series, he feels closest to being a blank slate hero in terms of design. I just find him boring.
Zoro: He's the most 'badass' designed character, which people who enjoy OP's silliness find boring. Luckily, since I have the opposite of that preference, he's probably the best designed of the crew for me. Post time skip gets a larger torso is an improvement I didn't expect. I think his scar with stitches look is shit, but other than that, I like him.
Usopp: Post time skip is clearly superior.
Sanji: All he ever dresses in is suits. He's boring.
Nami and Robin: Too sexually objectified for anything else to matter, really. It doesn't help that they're constantly changing their attire every arc, so while the guys havea "pre and post" time skip look with some variations, these girls get a new outfit every arc if not more often, so there's kind of an element of overchoice.
Chopper: His only good look was his monster form before he gained control of it. Chopper's monster form when he's not in control is terrifying. In control, he's goofy as fuck.
Franky: Bizarrely, I don't hate his outrageous look. Maybe it's because I was a fan of ace ventura when young. Either way, he's fun, but I think his post time skip is a downgrade.
Brook: I remember one fan art of him depicting Brooke's design more realistically and it looked really good. In OP, I hate it. I prefer his TimeSkip look. But I think it's more that I just hate OP's cartoony aesthetic.
Pudding: The greatest.