It's between Kuja and Kefka for me. Kefka certainly managed to succeed in becoming an all-powerful God and ruling the world as he pleased as well as making his power unchallengeable by anybody really, removing those that stood a chance of defeating him (the main party) out of the picture by separating them and forcing them to hide.

Kuja, on the other hand, successfully destroys Terra and kills one of his biggest opponents (Garland). One could argue that Kuja didn't accomplish a great deal personally because he often had pawns act for him. Zorn and Thorn acquired the Eidolons for him, Brahne destroyed all the nations bar one and the party gave Kuja access to Mount Gulug. However, I think that Kuja's ability to use these people add to his accomplishments in that he achieved so much without personally putting himself at risk or getting his hands dirty. Kuja managed to ruin the Mist Continent with Cleyra and Burmecia arguably being beyond repair, challenge Lindblum's position as the most powerful nation and get within mere inches of destroying all life as he had hoped. If I'm going to be lovey-feely I could even say that Kuja's eventual redemption by saving the party and realising the error his ways encompasses a pretty big moral 'accomplishment.'