Hey Kudistos, while it is pretty stupid saying "my character would beat your character in a fight, mine's better"(I'm pretty sure that's some form of ad hominem), and I agree that simply how powerful or impressive they are has little to nothing to do with how interesting they are on its own, when analyzing characters like Kefka and Sephiroth, using power as a point of quality comparison isn't totally meaningless, if you do it right. The thing is, creating a narrative context where a character achieving a certain degree of power is believable, while still being exceptional in the universe in which it's set, does require some skill, and does help add to audience fascination. And if surprise at such power can be created, without hurting belief, it can sometimes reach something close to awe.
See in this sense, though, I think that Sephiroth wins. While Kefka at his most powerful probably would beat Safer in a fight, I was far less surprised when he became so powerful. Sephiroth, on the other hand, combined impressiveness, belief, and surprise, all so strongly that I feel his transformation is the only instance in gaming or any medium where I can comfortably omit "something close to" from my last paragraphs final statement.
Of course, disregarding surprise or belief, clearly God is the best character ever. Fucker drowned the whole planet and somehow fit 2 members of ~14 million species on a mid-sized boat just by deciding to. Can't beat that.