In a normal fight, Exdeath would defeat Kefka with his barrage of spells he used against (SPOILER)Galuf and some other powerful moves he has. If Kefka had the power of statues on his side and Exdeath was in his tree form, Kefka might actually win, but if Exdeath was in Neo-Exdeath form, he'd totally ruin Kefka by devouring him into the Void. I don't think Kefka's Fallen One would work against Neo-Exdeath, though I also think Neo-Exdeath's Grand Cross wouldn't work against Kefka (final bosses seem to be immune to most status ailments).

Story-wise, Exdeath defeats Kefka. Game-wise, Kefka's angel form is pathetic. 60,000 HP makes it easy to one-hit-kill him, but I can't see anyone taking away Neo-Exdeath's 200,000+ HP too quickly. Exdeath wins here too.