When it comes to overpowered, nothing beats obscure Japanese titles...
Gunshin Kyoshuu (War God) Demonbane, the largest mecha in published fiction. So enormous it popped the universe it was in and destroyed other universes by accidentally brushing against them, and yet, is weaker than...
Elder God Demonbane. Older than existence, rewrites all the rules of reality at its whim, fights and wins against omnipotent multiverse creators, and is capable of summoning all versions of itself that ever existed, exist, will exist, never existed, does not exist, never will exist, and cannot exist.