1. Goku, from the
Dragon Ball/Dragon Ball Z universe
2.
Dormammu, from the Marvel Comics Universe, Earth-616
Since Superman has little to no immunity against magic that I know of, I would suspect he would also be weak against demonic powers, and Dormammu has sorcery based powers, immortality, is immune to physical and mystic attacks, and his body is composed of pure energy, so he would most likely put up a challenge against Superman on Earth, but I still think Superman could find a way to defeat him, but if Dormammu could find a way to fight against Superman in his own realm, The Dark Dimension, Supes doesn't have a chance.
3.
Q, from Star Trek: The Next Generation
Q is a member of the Q Continuum, and Q isn't really his real name, just an alias they all go by, even though I'm not 100% sure on his limits of his powers, and I know Q isn't omniscient, but he may be omnipotent, because any of his thoughts can become reality. Members of the Q Continuum do not abuse their powers like "Q" does, but I feel like Q could easily kill Superman by himself,
I'm just ignoring Pwn's post.