I found Kefka and Kuja the least impressive out of all the FF villains I've encountered. They're certainly similar in verbosity and flamboyance, which makes them interesting characters. However, their hatred - their reasons for wanting to destroy all creation - are so flimsy and arbitrary that they just aren't as compelling as other villains. I prefer it when the antagonist is driven by some powerful cause, because it makes them more convincing and more frightening - insofar as a make-believe nemesis can be convincing and frightening. Sephiroth, Vayne and Zemus (among others) are all motivated by profound and world-shaking causes, whereas Kefka and Kuja basically just say, "well, I've got all this power and life isn't really meaningful, so I might as well destroy everything."

I'm not saying they totally fail as characters. Kefka provides great commentary on the pointlessness of futile struggle and useless power, while Kuja's got a lot to say about mortality and finding meaning in life. But as villains, their desire for destruction is just so aimless and shallow compared to some of their counterparts. They have fairly common anxieties, which are turned into universally destructive potential when the plot imbues them with god-like power. The other villains aren't that simple...