Leader: Ramza, the best in the series. I generally have him as a ninja, but you could give him Calculator secondary ability with all magic learned to really smurf some baddies up.
Damage caster: Definitely Terra, with her extremely powerful magic.
Healer: As much as I dislike Aerith, what little she does, she does very well.
Ranged fighter: Maybe Rydia, because you can never have too much magic. Unless you mean physically ranged, in which case Sazh.
Melee fighter: Orlandu. Actually I could just send him in against pretty much any opponent.
Wild Card: Cid Highwind, because sometimes we just need to sit our asses down and drink some goddamn tea.