IIRC, at one point some cosmic being or other ( the Watcher maybe ? ) summoned the smartest mortal in the universe and got Reed Richards, who didn’t seem surprised that he qualified.
Is Doom’s disfigurement via college-era failed experiment still canon? If so, end of story. Reed saw the errors that Doom did not, and that was even before the cosmic-ray bombardment that Reed himself has speculated to have boosted his intellect.
Pym is a scientist; Stark is an engineer. Give him a little time, and Stark would build a better Ultron more cheaply. Or if he’s on a government cotract, he’d add extra features on a cost-plus basis.
I don’t think you all are really thinking this through. How does Doom defeat Vader? Any projectile or energy weapons would be deflected by the light saber. If Doom destroys the ground underneath him, Vader flips through the air to safety. Magic? Against a Lord of the Sith? Not happening.
Doom, of course, is similarly impregnable, so they would fight to a standoff. The scene writes itself: