Just beat the game less than an hour ago. First time. I used:

(These levels are approx.)

Zidahn: 51
Garnet: 50
Amaranto: 45
Quina: 40

Thank god for Quina's auto-life. That saved my ass on the final boss so many times.

I like this game (didn't like 1 or 10, have not played 3-8, or 11, love 2 and X-2) but I don't think I'll play it again. (Next on my list is FF Anthology.)