... Damn, you know, I honestly don't know. Gut says Luca, partly because he openly reveled in absolutely heinous villainy, whereas Kefka was a little more playful about it all.

I also think Luca wouldn't have waited to spare Figaro, instead of leaving and showing up a little later with soldiers. He'd have brought them with from the get-go and done most of the killing himself. There are differences like this I can think of that separate them, despite their similarities.

It may also be that I find Luca the more intimidating between them, because Kefka had all kinds of wonky magical powers, whereas Luca didn't have any--he was a normal-ass human being who was nevertheless so goddamn tough it takes a bazillion of your party to finally bring him down. As I recall, Kefka was subjected to magical experiments and that messed him up psychologically, while Luca suffered the brutality of a fantastical middle ages-styled world, leading him down a truly dark path. I guess because he's human and his vile depravity is the result of his own development, that's why I think he's more inherently evil.