if it was so obvious we wouldn't be having these discussions.
i dont think you've thought much about neo X-deaths powers.
i cant properly decide but in the final battle, kefka has powerful attacks and neo X-death has all flashy universal destruction attacks like grand cross and ultragust.
all the magic in the world VS the ultimate power. i think its quite clear that we'll never reach an agreement that we can all live with though.
seems as their from different games it's usually unfair and pointless to decide who is the most powerful.