Id definitely say the marvel universe. Theres tons of powerful abstract entities he would hav to go up against and i doubt very much that he could defeat them all singlehandedly. You've got galactus, Death, the Phoenix force, the celestials, the cube beings. All of those beings mentioned hav universe/multiverse destroying power.
Add Oblivion, Love/Hate, Galactus, possibly even full-powered Franklin Richards could help out (though the abstarct beings might make a better fight of it than he could as just a wicked powerful mutant).
Any opinion is just based on theories and assumptions in this thread but I'll give the win to MU.