Ultima's my personal favorite, but I wish you could use it as well as (SPOILER)Kuja's Ultima attack on FF9. It was a tad anticlimatic in FF7 and FF Tactics. But it's a cool attack spell, and tends to have a short animation (as opposed to some spells/summons that take too damn long).

On the defensive, probably the only healing spells I bother with late in the game are Regen and AutoLife, and possibly Phoenix if I'm in big trouble. For the rest I use items or simply kill everything (with Ultima) before I get too banged up.