Kane is one bad dood. Hell-bent on taking over the world, this general generally makes your life a living hell by removing Einstein from history, and leading the NOD forces on a campaign of world conquest. That doesn't sound very fair.
The Ghost Pirate LeChuck comes in at number 9. He's a zombie. He has a ghost ship that sails inside the lava which you get too by going into a giant monkey head. ("That's the second biggest monkey head I've ever seen!") He also tries to steal Elaine Marley from our good pal Guybrush. Pretty selfish, if you ask me.
Sephiroth was the result of a failed experiment which was carried out by the Shinra Corporation. Son of Jenova, Sephiroth posses super-human strength, magic that rivals that of any warlock, and a big ass sword. He isn't the kind of guy who you would like to meet in a dark alley; he sure makes stuff difficult for Cloud and his crew.