I had this discusion a while ago.... And Kefka would by far scare me the worst.

Sephiroth probably wouldn't do anything thing to me unless I was in his way... and then I could expect a fast death more hten likely.

Vs.

Kefka.... he would scare my pants off... sure in the game he is quite a pushover... but he is off his rocker and you never know what he would do.... particually if you made him mad.... heck even if you were his ally you couldn't guarante yourself safety. His insanity and clownish face and evil laugh make him scary..... and funny... but don't laugh to his face.... that wouldn't be smart.