Mxy is very powerful. One of the most powerful beings in marvel. There is a respect thread only for him. To answer you question, he could go toe-to-toe with Thanos and his IG. Though, that is just my opinion. Others will say no, but he isn't far behind.
In his respect, he casually destroyed universes for his own amusement. He has destroyed the multiverse and put in back together in his own image with a snap of his finger. He even beat the top dogs in DC like Spectre (who was very powerful at some encounters).
there is no limit to his power.he's far more powerful than IG,proably equal to if not more powerful than HOTU.His absolute most powerful was Wrold's Funnest,but since people don't accept it as canon,Emerpror Joker is probably the most powerful canon example ur gonna get.he's only under pre-retcon beyonder(WF is equal to him imo) in power
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it's good for the bones
part of a balanced breakfast!
there will always be debates concerning those questions, but mxy's feats speak for themselves..
-easily destroying/recreating the multiverse [along with even conceptual dimensions, like hell.]
-showing absolute control over time/reality.
-owning spectre repeatedly.
-shrugging off blasts from ultimator [of whom was owning other 5-d imps, with ease.]
-demonstrating the omniscience to go along with his omnipotence.
-retconning specific characters on a whim.
-breaking the 'real world's' and marvel's 4th wall, whenever the hell he feels like it.
-venturing into, and out of, the source itself... just because he feels like it.
-etc.
in short, mxy has always been an excuse for writers to do whatever the hell they want.
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"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."
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I would say yes Thanos IG can go toe to toe with Mxy...the fact that the IG makes you all powerfull in these areas in particular...mind,reality...and we know the rest...to me it shows that concerning an actuall fight...i dont think thanos's mastery over reality will make him go down quick....so yes he can goe toe to toe with mxy at least concerning himself and what can be done to him
Concerning an actual vs match...i would give mxy the majority...as Glaan has said...his feats are superior