I dunno, i agree about Sephiroth,
sure Kefka and Kuja are more metal and perphaps more evil but they weren't as, i dunno, spehiroth just has something
He's bad but, he was good once, he did some messed up stuff and you feel his presence althro the game, i remember seeing that Serpant chiskababed and i thought "OHHHH he's been here!"
Its the style, the sword, the fact that he was product of one of the skankiest 3 ways ever, and the fact that he murdered a main heroine and kicked off one hell of an adventure