For now, I use Quistis, Rinoa, and Squall... and to be honest, I'm not sure why. I know I use Quistis for insane experience rates (degenerator + Island Closest to Hell), and Squall simply because I LOVE Lionheart, but Rinoa.... I guess it's because I used her against Ultima Weapon and a few other bosses for Invincible Moon, and decided not to switch back to Zell (for now). Now those three are level 100, Zell is in the 30s, and Selphie and Irvine are both 15. Heh, I have some work to do, because I plan to use Irvine and Zell against Omega...