Actually the most powerful character ever written in fiction is the God of Text from Suggsverse.
Just look at the Suggsverse cosmology here:
The God of Text is even above all of this. The God of Text also owns and controls all of fiction, including fan fiction. Basically anything that happens in fiction is only because he allows it to happen. So you can't even create your own fan fic to beat him.
Batman with prep can beat him.
God of Text owns Batman's story. Anything that Batman does is only because God of Text has it happen. Even if Batman beat GoT, that would have only been GoT's fictional version inside of GoT's realm. God of Text exists outside of all fictions (which he contains in his realm).