I think Omega is referring mainly to the amount of damage caused by an attack in battle, and Kuja's Ultima attack dropped all of your characters down to 1HP, if I remember correctly. There are certainly more powerful attacks than that. Few that are cooler, but certainly a lot that are stronger.
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.
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