People only think Kefka is a great villain because it reminds them of Joker, and the truth is a lot of people don't even know Kefka's true origin.
Kefka is thirty-five years old and Emperor Gestahl's right-hand man. He joined the Empire and if anything is he was just an above average soldier that rose through the ranks. At least sixteen years ago Kefka was the first experimental Magitek Knight, which gifted him magical power, but shattered his sanity as the process had not been perfected yet. Kefka became a cruel, destructive madman, and acquired a reputation as one of the most dangerous men in the Gestahlian Empire. Gestahlian thought it was a success. About six years after the procedure, the imperial military was restructured with Kefka and Leo being involved.
So you see Kefka was not some dude who had some dream and came up with strong long and in-depth plan to be come the all mighty God of Magic. He's not a rags to riches. He was literally brain damaged and went insane and re-imaged everything to be destruction and the Empire rode his wave. He literally YOLO everything. Stop tryna make it sound like he was something more than he not.
"B-b-b-but he became the God of Magic and destroyed the world!" Yes he did but he just pushed some statutes around. It wasn't even some master plan. He was just there and decided to do it out of desparation! Like I said, he YOLO'd everything.
"B-b-b-but he killed an entire kingdom by poisoning their water supply! THAT'S VILE AND MAKES HIM THE BEST VILLAIN EVAR!!!!!!!" Yes that is very vile and a very villainous thing to do, but that alone doesn't make him the best. Other villains has done similar things such as destroyed an entire town, do we call them the best villain just for that? Sheesh....
"B-b-but he actually succeeded in taking over the world no villain has ever done that before dude" Yes he did but again, let's look at the context on how he did it. He didn't start off as some almighty strong person who did what he wanted, he was just lucky in his situation. At least Joker was a criminal genius, manipulative mastermind and a genius in biochemistry. Kefka is nothing close to that.