What about kuja's Ultima attack...he whiped out a whole planet with that.
Now if you want to talk about destruction, Sephiroth's apocalypse, or Ultimecia/Griever's move Great Attractor, eliminates four massive planetoids, and Earth. Also I though a really sweet move was the enemy skill, Pandora's box from FF7. You get it from the Zombie Dragon in Sephiroth's Crater.