Gunner: Yuna here. Not really because it looks good but because it's her iconic dress and thus you get used to seeing her like this.
Thief: Rikku here because the other 2's skirts are god awful + Rikku iconic look.
Warrior: Even though this is Paine's iconic look, Rikku looks way better.
Songstress: Rikku again. Yuna is too frilly/fluffy and Paine is just awful.
Black Mage: Rikku because zettai ryouki.
White Mage: I like Paine the best here because she makes White Mage look fun instead of holy/pure/boring.
Gun Mage: Rikku here as I don't like Paine's pants and Yuna is too purple and her sleeves are stupid.
Lady Luck: Rikku because fishnet stockings + garter-belt.
Trainer: It's a close battle but I'll give it to Yuna.
Alchemist: I'm not a fan of the shoulder pads but Yuna still looks best here. Nice gun too.
Berseker: Sometimes, the only way to win is not to play...
Samurai: Yuna's skirt is awful, Paine's pants too so the win goes to Rikku.
Dark Knight: Paine looks the best overall even if she has way too many 'wings' for my taste.
Mascot: Burn this fucking filth with the fire of ten thousand suns.
Festivalist: Classic kimono Yuna is best.
Psychic: Yuna's costume is pretty cool and her visor is badass.
Special: Rikku's machina maw as the other two just don't do anything for me.
Well, looks like Rikku wins by a landslide for me.