Kefka was undoubtedly superior to all of the FF villains up until that point (I actually prefer X-Death and Golbez, but whatever…). However, while his antics were entertaining and sadistic, he is ultimately surpassed by later antagonists. Kefka’s 2-D qualities work rather well in FFVI, but IMO the more complex villains such as Kuja, Seymour and Sephiroth win out.
Then again, the Deus Ex Machina villains such as Neo X-Death, Zeromus and Necron completely own these guys as well, so meh…