Sephiroth. At least he had some depth. Kefka was more entertaining, but as a character he was just "mua-hahahah-eeeeeevil". That and he didn't have an iconic badass walking-into-the-fire scene like Sephiroth.
Whatever depth he might have had has been thoroughly murdered by the millions of fangirls who drool on his feet nonstop.
Ah, with me it was the opposite. Back in my Usenet days I grew thoroughly tired of the Kefka fanboys who always used to go on about how much better FF6 is than FF7 blah blah etc. Yeah, FF7 fanboys can be pathetic, but I've seen the flipside of the coin and it ain't purty either.