Im gonna say One-Above-All is the most powerful fictional character ever.

He is literally omnipotent, omnipresent, and omniscient, and is above all cosmic powers and abstract entities.

He can manipulate his size and mass, has telekinesis/telepathy, can teleport, can bring back the dead, can GRANT immortality, and that is just a few things mentioned. This guy is literally unbeatable. He can manipulate reality in real time.