In the most infamous moment of Marvel's Secret Wars event, Thanos' skeleton from his body, supposedly proving who is the superior villain. However, while Thanos' death seems to declare a definitive winner, the events that followed actually suggest he won the confrontation.
Secret Wars was Marvel's big event of 2015, and it brought to fruition years of storylines teasing the destruction of the Multiverse through the Incursions created by the Beyonders, creatures of almost limitless power existing beyond reality. The only man in the Multiverse capable of stopping them was Doctor Doom, who allied with the Molecule Man and Doctor Strange to destroy the Beyonders, even though it was too late for the Multiverse. Doom, however, managed to absorb the Beyonders' limitless power and salvage what was left of reality, creating a patchwork of many worlds called Battleworld, where the survivors of the cataclysm lived on. God Emperor Doom ruled as the absolute master of that world, until two vessels from Earth-616 arrived, one carrying a group of heroes, and the other the fearsome Cabal, led by Thanos.
Thanos is used to holding absolute power, having obtained it through various means over the years, including the Infinity Gauntlet. He cannot stand Doom being his superior, so he sends out his Cabal to raise an army against the God Emperor, even awakening the Fantastic Four's Thing in a gigantic new form to march on his floating castle. After unleashing zombies, androids, and giant insects on Doom, Thanos faces him personally, despite knowing he wields unimaginable power. It's at this point that Doom destroys Thanos, and yet this isn't where the conflict ends.
At the end of Secret Wars, Doom is no longer a god, Thanos is restored to life and his former power, and the Multiverse is remade. Doom's fall comes at the hands of Reed Richards and the Molecule Man, but without the distraction provided by Thanos' army, Reed would not have been able to reach Molecule Man and find out the truth about Doom's power. While Thanos doesn't depower Doom personally, it's the goal he works towards and one he succeeds in, ending the event resurrected and free to pursue his goals. While he may not win the battle, Thanos wins the war, and is an essential part in overthrowing Doom.
It's also worth considering that death doesn't mean the same thing to Thanos as it does to anyone else. Thanos has been described as allergic to life, and his fondest wish is to finally Mistress Death. To Thanos, death is a paradise, so even being killed by Doom can't truly be described as losing, especially when the distraction he caused was instrumental to stripping the villain of his power. This could explain why Thanos is so confident when facing down Doctor Doom despite their power discrepancy - he doesn't actually need to 'win' to get everything he wants.
Looking at their confrontation in this way helps explain Thanos' apparently foolish behavior in Secret Wars, which was one of the most controversial points in the series. It's unclear whether Thanos believed he was a match for Doom or truly knew exactly what he was doing, but either way, the event ends with Thanos restored to life and power, and Doom cast down from his godly position. Doom ripping out Thanos' spine was surely an epic moment, but in the end Thanos got everything he wanted and Doom didn't, meaning that their fight did not end the way everyone thinks.
