This opinion is subjective, but I highly feel and think that Kefka is a better villain than Sephiroth. In terms of story-telling, writing, and villainy, Kefka wins by a landslide. I think Kefka is one of the most well-written villains in all of gaming, and he is iconic for that reason. However, many people seem to love Sephiroth more because Final Fantasy VII became more mainstream than Final Fantasy VI. Regardless of brute strength or who would win in a 1v1 battle, in terms of character, Kefka takes the cake. Plus, he literally became a God, which was his main goal throughout Final Fantasy VI.