Eh, I'd say Knights of the Round, FFVI... Damage wise, it does 129,987 HP damage. If you don't consider that a move, but rather a spell, then I don't know, since I haven't played all the FF's.

Actually, with the right Materia line-up, you can mimic the spell, and do twice that, or 259,974 HP damage, which is done to all your enemies on the screen (Or so I believe it does...).