This is pretty difficult.

Quistis- Second in Command of the Team. Useful magics and strong leadership skills.
Garnet- White Magic and Summons.
Rydia- Black Magic and a backup Summoner.
Beatrix- Leader of the Team. She'd take down the magic resistant enemies and defend the casters. Plus if Garnet fell she could cast White Magic. Also has powerful Sword Techniques.

So, yep, my Ultimate team.