X-Death would probably beat Kefka by battle mechanics because of Flare + Holy + Meteor in 1 turn.

Story-wise, Kefka wins. Yes, because even Normal Kefka was able to kill 8 Espers at once. Doubt X-Death could do that.

As for NeoX-Death vs God Kefka, it's hard to decide. NeoX-Death could send Kefka to the Void, but because Kefka controls all magic in the world/universe, he could just get himself out again.