Surprised not to see the Moogle/Tonberry/Cait Sith outfits on here, I particularly love Paines Tonberry.

my personal top 5 (done by person) would be:

Yuna:
1). Floral Fallal
2). Moogle (Mascot)
3). Dark Knight
4). Black Mage
5). Songstress

Rikku:

1). Thief
2). Warrior
3). White Mage
4). Berserker
5). Trainer

Paine:

1). Full Throttle
2). Tonberry (Mascot)
3). Warrior
4). Dark Knight
5). Black Mage

With Paine number 4&5 are very close, I love both but the skills of DK make me lean towards that.