Luffy :
Onigashima
Zoro :
Skypea
Nami :
WCI
Either the oktoberfest outfit as posted before or the wedding dress
or the first W7 one with the tie
Usopp :
Don't really have a favorite, I guess FMI because he actually looked like he got stronger with that muscle definition
Sanji :
His first WCI outfit, looked like an actual prince
Chopper :
Enies Lobby
Robin :
Miss Allsunday
or Skypea
I agree with
@moreha9685 for cowboy hat fitting Robin the best
Franky :
Water Seven, thug Franky is best Franky
Brook :
Thriller Bark
Jinbe :
WCI, i guess?