For me it is the ladies and I will explain why:

Fang is my party leader due to her having the ability, Highwind, it does massive damage due to Fang's high STR stat along with staggering an enemy.

I use Lightning as support as a Ravager and a Commando, Ravager to help stagger for Fang to kill, and Commando to help beat on enemies until the stagger gauge drops.

Vanille because she is a great Medic and helps out in a pinch, and I also use her Sab and Syn skills as well.

I apologize but I have beaten the game so I have moved on to their other roles, aside from the main 3 they start with.

The reason I use this party is for Adamantoise farming, I need Fang's Highwind ability to one-shot the legs to bring the Adamantoise down, so I can debuff it and beat it to an easy win =]