I completely disagree. (SPOILER)Kefka had really no desire to control the world, it just sorta fell into his lap. The only thing he enjoyed doing from the moment he was introduced until the end of the game was just murder and destroy everything. Sephiroth ultimately wanted to become a god by destroying the planet, Kefka became a god and decided existence was utterly meaningless so his last action as god was to annihilate everything until the party put a stop to it.
Kefka is also the only villain to take pure pleasure out of the suffering of others and he does so indiscriminately. Whereas other viallins always had a focus for their rage (Cecil, Cloud, Zidane, etc...) Kefka seriously just kills people cause he enjoys murdering and torture. Every other villain always had some "end justifies the means" excuse for their actions. Golbez destroyed countless kingdoms to get the crystals and manipulated Cecil and Kain to do it for him, Ex-Death destroyed countless cities that were the strongholds of the forces opposing his plans, Sephiroth killed anyone who got in the way of his plan or used it as a means to keep Cloud and party on his trail, Kuja killed people to draw out the power of the Eidlions and Trance so he could defeat Garland and most of the other villains have similar reasoning behind their evil actions.
Kefka tries to burn down the capital of an ally kingdom just to retrieve a fugitive, beats the living crap out of two espers who were already dying just for the fun of it, burns down Thamasa and murders countless villagers and his own "comrades" just for the fun of it despite having little to do with his mission of retrieving Magicite. He keeps the entire world in a state of constant fear as he rules like an evil god destroying entire villages on a whim. He says even himself that destruction is pointless if precious lives are not lost. No other villain shows this much maliciousness in either their actions or their dialogue. He kills simply cause he enjoys it and has no real desire beyond that.
Kefka is amusing cause he's both a terribly malicious villain but also very likable. He'll make you laugh at least once which is more than all the other villains who take themselves seriously can say. I think that's what makes him a good villain, he's still an evil SOB but he's also very likable. How else do you explain how he has a cult in real life despite being in a game that doesn't have the sheer numbers of the later PS entries.
While he certainly is not the best villain in the series, I definetly feel he's one of the better villains. His backstory is incredibly simplistic though it hints at untold tragedy but serious lack of details makes him come across as a simpler villain compared to other FF villains.