Honestly, probably Ultron's head, if you think about it that would be Infinite Ultron's head, meaning the same Ultron that not only killed Thanos immediately but also: broke into multiple different universes, could tell that the watcher himself could see him, and could destroy entire planets with ease, so the fact that this version of Doom could beat him and all the others including Fortnite's supercharged Galactus is INSANE if you're talking about his power level.
