Batman-Prime
Kryptonian Scientist
It's always hard to compare "Cosmic" beings of different Universes.
Is LT as powerful as the Spectre? Is Spectre the most powerful being? If one company allows another being to defeat the TopDog does it mean it would be able to defeat the TopDog of another Universe too?
By Universe I mean the real Marvel or DC Universe, it doesn't matter if the particular Universe is an Multiverse or just one Universe, because the Multiverse/Universe represents always "ONE" Companybased Universe ala Marvel/DC.
Look at Krona, in the JLA/Avengers Crossover he destroyed whole Universes, adding thei power to his own, he treated to destroy the whole Marvel Universe/Multiverse, so we can say for sure that he was more powerful then both Spectre and the Living Tribunal, even TOAA.
Could the Anti-Monitor do the same? I don't think so. Is he even aware of the existence of another Universe/Multiverse outside of DC? Would his powers work under the Rules of the Marvel Universe? I guess not, and if one of those Rules is, that no being is beyond the TopDog, means LT, he wouldn't even be allowed to enter the Marvel Universe without permission. Why was he more powerful then LT's DC-counterpart the Spectre (who needed the help of the most powerful mystics to temporary "defeat" the Anti-Monitor)? Because the "Source" allowed it, it wanted the Heroes to accomplish this task (by it I mean the DC heads😉). Marvel could of course create it's own universethreatening Villain who would kick LT's ass, but it wouldn't be able to even touch the Spectre.
In my eyes, LT and Spectre, wield as much power as the TOAA/Source (or Brothers) of the particular Universe grants them to accomplish a task, making them more powerful then anything else. Without it they are maybe powerful but not even near the level they were before. So if the Anti-Monitor would be allowed to enter the Marvel Universe he would loose against LT but only if TOAA would allow it.
If LT would come into the DC Universe, Dr.Fate alone could defeat him 😛, the same is true for the Spectre in my eyes, he wouldn't last an minute against Dr. Fate for example in the Marvel Universe 😉.
Conclusion
LT > Anti-Monitor in the Marvel Universe
Anti-Monitor > LT in the DC Universe
Anti-Monitor > LT in a neutral Universe (means LT cut from his powersource)
As for Tiamut
Anti-Monitor > Tiamut
Cause he allready absorbed too much of the DC Universe/Multiverse.