I voted Kefka, though there's a few others up there that I did like, but crazy anarchists are just rarely played so I had to pick him. Plus he was the first FF villain who was just a guy, and remained that way. No supernatural powers to become a demon (Chaos, Emperor) or some sort of being from another world or of another race (Zande, Zemus) but just a normal human who found power.

As for other villains on that list. I kind of liked Garland. While not played so much in the game, he had a classic demon like villain going towards the end, achieving a lot of power to become Chaos.

Zemus was another great villain I thought. Although he appears late, I liked how he was introduced. It began that you saw signs that someone was above Golbez, until finally you learn that Zemus was a being from the moon, who thought himself god compared to people from Earth, and wished to wipe out humans so he can create his ideal world. He wasn't a god, but had a god-complex going.

Sephiroth was ok, not the greatest of villains. I never really liked how he got portrayed in the game. He just had an air of "I'm so cool" flashed a pose and did something flashy, then made you fight a chunk of Jenova. Though I'm somewhat bias, I hated his character design, I'm no fan of Nomura's work, but of all of 7's character designs Sephiroth was among my 2 least favorites. The whole Sephiroth villain was a good premise I thought. A person born of a scientific experiment who finds out his existence and decides to just destroy the world. I liked the premise, but not the execution.

Other notable villains I liked were some of XI's, but I'll spare people my thoughts on all of the numerous ones in the game.