While I'll admit the game does have plot holes, several of those have been filled in with other games. The main reason I like Sephiroth is the way he plays off of Cloud. I found Kefka to be very bleh, and while he may be unique in the FF series I've played plenty of other RPGs with villains like him. Besides, he just came off as a generic bad guy in my opinion. Hell I find Golbez from FFIV before he changes to be a much better villain then Kefka. While Sephiroth may not be unique he does play off of the main char, Cloud, really well and is not something I've seen done too often.