Kefka- damn near destroys the world, and becomes a god.
Kuja- Destroys a world.
Emperor- Uses cyclone of doom to destroy several cities, then takes over Hell (and Heaven, at least in DoS)
Chaos- Meh, keeps the world in darkness for 2000 years. That's gotta count for something.
Sephiroth- doesn't really stand out, power-wise.
Zemus- Didn't really accomplish all that much
Ultimecia- See above

And I can't really judge the others yet. I'm working on the other games, though.