Zoro's outfit in Dressrosa is so great, because not only he looks good in it, but it changed through the whole arc: from suit with moustache and sunglasses, all the way down to suit with bandana. There's something so great about a badass looking so silly.
Sanji's best outfit for me is whenever he wears glasses, especially at Alabasta and the beginning of Dressrosa. Maybe it's the fact that he looks silly too for wearing glasses with only one visible eye, or just how well they mesh with his haircut and brow and everything (also his Long Ring Long Island outfit with the ball on the head is pretty legendary).
Luffy's best outfit is probably either the Lucy one for originality, the afro one, the fish one, or the naked one. He just has so many weird ones and he looks fine in all of them, but my favorite is Lucy just because of how easily recognizable and memorable it is.
Nami's i think is either the Water 7 one, or the Thriller Bark one. She just looks so good with a skirt and a tie, or with a ponytail and with a bride dress too. Ultimately i would go with the TB one, just because the haircut helps making it even more unique and memorable, i wish we could see her again like that.
Chopper doesn't really have that many outfits, but the first one after the timeskip with the new hat is pretty cute, and that's all i can say about it.
Franky's Dressrosa outfit wins everytime, from the beard/sunglasses one, to the cannon style, to the last missing eye look he got, it's just been a good arc for him on all aspects.
Usopp doesn't have that many outfits i'm fond of him, but the Sogeking obviously stands out and i have to go with the easy one, it's just that good.
Robin's best one i think is either the very first one, or the Water 7 one, but ultimately the leather look suits her well with the dark hair and all.
Brook original outfit is easily the most iconic, but i really like his soul king outfit too for just how opposed it is to the original one.
It seems that Dressrosa's outfits are the majority for me, they are just so different and memorable and creative that thanks to the arc's longevity, they just give a lot of time to be remembered, and since they're great to begin with and they change along the way it's only a good thing.
As an honorable mention, i would say that my favorite outfit of the series so far has to be Doffy's in the flashback (look, another Dressrosa one :ninja:), the one with the suit and the pink coat and the gloves, it's just so gorgeous and it's a shame he ditched it for a more bland one in the future.