HMM... Tis a tough one indeed "Well they're all evil and psychotic." - Ryoji1337
I do agree with Ninja because Kefka IS the only one who completed his goal.
Kuja one the other hand definitly holds close competition but really he was only tough himself. Against Sephy and Kefka he is outnumbered because Kefka not only has control of the world but he also has that damned tower to fight too and Sephy has Jenova to aid him (not that he will needs it).
+ Kefka also has that infernal laugh which always sends a chill down my spine.
Kuja is powerful but I don't think he can cope to the might of Kefka n Sephy.
(Plus he's not even the final boss)
Even though Sephy has his shear power and agility and Kefka can often be known as a "wuss" I'd have to vote for Kefka because:
1 - He altered the entire world
2 - You had to use 12 people to defeat him. (Or at leats get to him)
3 - His laugh (again) is spine-chilling
4 - He just seems more like a "villain" than Sephy or Kuja
5 - 6 is my favortie FF
:toungesmile: Well... There ya go
<edit> Not necisarily you CAN go and fight Kefka with just 4 people Sabin, Celes, Edgar, and Setzer however it would not be easy