The best FF villain is ExDeath.
Kefka, Sepiroth? Destroy a planet? Some twisted tale of humanity's powerlust?
How about a magical tree that humanity used to seal evil spirits into until one day it became so corrupted by it that it said "Y'know what, I'm tired of humanity's crap," sprouted legs, built an army, and constructed a succesful plan to collide alternate dimensions to gain access to an interdimensional bypass to then be in a position to conquer all of time-space.
Edit: And when he kills, it isn't in some cheap backstab. No, he lets them have a one on one fight to truly feel the utter despair of how impossible their dream of striking him down truly is.