The problem with this topic is that the Marvel Universe has FAR too many powerful beings...on Earth, cosmic and mystic. I'll focus on Earth based beings first. Thor, Hulk, Doom, Sentry, Juggernaut ect...I think would pose a great match. If they could beat Superman is another story. Cosmic level would be Gladiator, Black Bolt,The Annihilators, Silver Surfer, Thanos, ect This level of beings have alot of power and have a much higher chance of beating Superman. Im not going to rattle off the reality warping people nor the stupidly powerful like Galactus and The Living Tribunal and Beyonder and so on. Mystic is a tricky category because we dont know if the magic of Marvel Universe would affect Superman like DC Universe magic does...so no comment. All in all I think the best way to see who in the Marvel Universe could beat Superman is measure those who can beat The Sentry / Gladiator and take it from there.