VII is my favorite but I really enjoyed VI.
I think having played VII first and absolutely loving it I just compared every other game to it afterwards, and as a result none seemed as good due to the fact that they were different.
I have to admit though, that Kefka > Sephiroth in the way that I actually hated him when I played the game (Although I thought he was a she) however Sephiroth has the wonderful background story that drove him to where he was.. Most of which is still a mystery to me.
Let's just agree on the fact that they both > Kuja and Seymour.
^.^