Here are some potential contenders, with caveats:
* The One Above All: The ultimate being in the Marvel Universe, literally the god of all creation. The Anti-Monitor is a multiversal threat, but The One Above All is beyond that, existing outside of reality itself. This is a guaranteed win for The One Above All, but it's not a fair fight.
* The Living Tribunal: A cosmic entity that acts as the judge and upholder of the balance of the Marvel Multiverse. The Tribunal has vast power and can manipulate reality itself. While the Anti-Monitor is formidable, it's plausible the Tribunal could find a way to contain or weaken him, though it wouldn't be easy.
* The Beyonder: A powerful entity from "Beyond" that has existed since the beginning of time. The Beyonder is essentially a god with reality-bending abilities, comparable to the Anti-Monitor's power. This is a close call, but the Beyonder might edge out a victory.
* Galactus: A cosmic being who consumes planets to sustain his life. Galactus possesses immense power and cosmic energy manipulation. While the Anti-Monitor is likely more powerful, Galactus could potentially find a way to exploit a weakness or use his cosmic powers to gain an advantage.
* The Celestials: Ancient cosmic beings responsible for shaping the evolution of civilizations. The Celestials are incredibly powerful, and a team of them might be able to fight the Anti-Monitor on equal footing. However, a single Celestial is probably outmatched.
Important Considerations:
* Cosmic Power Levels are Unclear: It's hard to quantify the exact level of power that separates these characters.
* Cosmic Entities are Not Omnipotent: Even the most powerful entities in both Marvel and DC have limitations.
* The Anti-Monitor is Extremely Powerful: He's a multiversal threat who destroyed entire universes.
Conclusion:
While the Anti-Monitor is one of the most powerful villains in DC, a few of the most powerful beings in Marvel could theoretically stand a chance. However, it's essential to remember that cosmic power is often a fluid and ambiguous concept. The exact outcome of any battle would depend on a multitude of factors and the writers' interpretation of character capabilities.