Sepheroth, no contest. Not only does he have more than 1 form, but he is aslo one of the strongest characters in the Final Fantasy Universe. That Masamune of his will rip through anything and everything.
Btw, dont consider any Final Fantasy made by Square-Enix a real final fantasy game. All the new ones are garbage now. They lack good storylines, gameplay is mediocre and there is little to no character development. Square-Enix is trying to milk FF for all its worth. Its just not right.
Now, if you were to pair these two up: Id vs. Sepheroth, then we would have a real fight here (Id is a character from Xenogears, also made by Square).