chaos: I've seen many other threads like this, but ah well. A fresh one may always help us.

Trowa: The hardest Final Boss for us would probably be Neo Ex Death as well, and the great, yet tense music only made it worse. I found his first form a bit weaker than the other form though, but still, a powerful foe.

Rubedo: Second to that, the first time I tried out Sephiroth, he was quite hard. Then again, that was before the time I started running around in circles levelling up for a few hours.