Now it's just a matter of taste.. I think it's a lot easier to hate kefka for his sadistic nature... He just wants to destroy everything, for his own amusement.
I say he's a great villain because he was an idiotic coward... He wasn't an all powerful, scary badass that threatens your life (BTW, Golbez did this job much better than Sephiroth)...
Again, Kefka is great because he is unique.