Enyalus
MALE DOMINANCE!!!
Doom's stolen Galactus's power and Beyonder's power, using only tech. And in a What If, after he takes the Beyonder's power, he also grabs the Infinity Gauntlet (unable to be used here) and solos the entire Celestial race.
So, he's gotta win.
If we're allowed to use certain artifacts, though, Thanos had the Cosmic Cube which allowed him to replace Eternity. He also had the Reality Gem which can create entire universes with its power.
Dr. Strange was able to beat and absorb a being who had infinite power (Aphion, or something along those lines?) One of Shuma-Gorath's upper level demons. Became so powerful that once out of Shuma's realm he had to burn off some of the power he had absorbed or he'd have destroyed galaxies by merely setting foot in the 616 universe again.
Oh...also, even though Doom has the better feats with prep than Thanos, Doom would never win against him heads up in a prep tourny. Thanos is one of the only humans Thanos feels the need to monitor. He's got pretty much all of Doom's facilities bugged. Anything Doom would do, Thanos would know about.