I'd actually put Kuja as better than Kefka. Sephiroth's definitely the best though, he was genuinely quite a threatening character throughout most of the game.
Very unlikely they'll match that. The thing they need to remember is you need to make your villain actually scary. With Sephiroth they succeeded with the trails of blood and Midgar Zolom impaled on a tree. That stuff really helps build the dread.







