Pep Up/Transfusion, which kills the caster to restore all HP and MP to an ally, is a favorite of mine. The first time I played FFV, my preferred strategy late in the game was to make everyone Summoners with Blue Magic, using Bahamut to attack, Transfusion to restore MP and Phoenix to revive people after they cast Transfusion. Not the most efficient way to play, but it worked.
The ones that affect enemies with specific levels are cool, especially Goblin Punch.
I don't think it was in any other games (maybe the MMOs?), but Quistis' Degenerator is great, an instant death skill that works on almost anything, even a few bosses.