Kefka vs. Sephiroth

I vote Kefka mainly because I didn't like Sephiroth after he went crazy, he just wasn't nearly as interesting as he could have been. He kept the story going but as a villain he was actually a bit boring, were it not for Cloud Sephiroth wouldn't have had much purpose, not to mention that Sephiroth didn't technically become a god-like being yet, he still needed meteor to "nearly" kill the planet so that he could become one.

Kefka was manipulating practically everything from the start, he was a supremely treasured strategist and you actually see that during the events of the game, considering how much power he had over Ghestaul, plus Kefka did actually become a god.

My last reason for choosing Kefka over Sephiroth is the aspect of their deaths.
Sephiroth (not including anything after the events of ff7 (especially since none of it even made a tidbit of sense, at least I thought it was ridiculous)) did not impact anything with his death. He died and the one thing he did do was destroyed by the one person he should not have killed. Beyond the destruction of Meteor, nothing changes in the world when it comes to Sephiroth's death.

Kefka isn't the same on that one. Kefka's death was immediately impacting on the world; magic was destroyed the moment he was. All the espers that may have been alive at the time, fade from existence, terra's life was also threatened but she had a strong enough connection with the children of Mobliz to keep her existence. With Kefka gone, magic does not exist and never will exist again, that has a lot of impact on a world.