Most of the FF villains are farily generic. Kefka was probably the most successful of the villains as has been pointed out he definetly did the most damage and the most evil next I think only Yu Yevon can compare to how bad Kefka was. Yu yevon was probably worse because his rampage lasted 1,000 years not 1. But Kefka is a fairly boring Character. He doesn't have a very compelling back story and his personality is frankly a flat out Joker rip off.

Now I think Seph is just the opposite He and a compellign backstory and a decent motivation for what he was doing besides "I'm evil and crazy" but he wasn't very successful at it, and he really at times; came across more like a whiny mommas boy then some terribly evil threat.