Dark Knight x2, Alchemist is a pretty good final party, but really it's up to you. I currently have 100% mastered on everything except Mascot, and I have each girl with a variety of dresspheres.

Yuna: Gun Mage, White Mage, Samurai, Black Mage, Mascot
Rikku: Thief, Samurai, Mascot, Alchemist, Berserker
Paine: Dark Knight, Samurai, Mascot, Black Mage, Trainer