Opposite of the other thread, and when I think about it, we don't do this topic too often. To make this a bit more challenging, don't count Yu-Yevon and Sephy in Cloud's head. They are more story based than actual gameplay based final battles.

My vote is a toss up between Kefka, Sephiroth, and Braska's Final Aeon, because it's possible to wipe all three of them in a single round with the right set-up and even without the uber gear/abilities, they're not terribly difficult on their own compared to other fights within their own games.