Deadpool literally can't die, but he also can't hurt Thanos — the Mad Titan wins through total physical dominance.
There's a fascinating wrinkle in this matchup from the comics: Thanos actually cursed Deadpool with immortality out of jealousy (both were in love with the cosmic entity Death). So Deadpool literally cannot die — but that doesn't mean he can win.
Thanos at base level can trade punches with the Hulk and tank energy blasts from cosmic beings. Deadpool's peak damage output is 'really good with guns and swords.' The damage gap is astronomical. Deadpool's katanas would bounce off Thanos's skin, and his guns would be less effective than a squirt gun.
Deadpool's healing factor is the only thing keeping this from being a one-second fight. Thanos would obliterate Wade repeatedly, and Wade would regenerate repeatedly, and neither could permanently defeat the other. But in terms of 'who wins the fight,' Thanos dominates every exchange.
The irony is that Deadpool's immortality — the very thing that keeps him in the fight — is a curse FROM Thanos. The Mad Titan is simultaneously the reason Deadpool can't lose and the reason he can't win.
Deadpool cannot die. His healing factor regenerates from literally any damage — decapitation, incineration, atomic dispersal. He's broken the fourth wall to annoy cosmic entities into submission. His unpredictability and refusal to stay dead make him impossible to permanently defeat.
Thanos is so far above Deadpool physically that this isn't a fight — it's Thanos hitting Deadpool until he gets bored. The Mad Titan's base strength, energy projection, and cosmic durability mean Deadpool can't actually hurt him. Thanos could also potentially lift his own immortality curse to finally end Wade.
Thanos 'wins' in every meaningful sense — he dominates every exchange and Deadpool has zero path to actually damaging him. But Deadpool 'can't lose' because he literally cannot die. The real loser is whoever has to watch Deadpool quip through being atomized for the hundredth time.
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