Thanos doesn't need to power drain the Surfer to beat him.
Galactus, Watchers and the Beyonder
Thanos has fought Galactus with his own regular power. And he's attained power far greater than what any of them possess.
his having outright conquered Marvel Earth and given it back out of boredom
Source? Or is this that same What If? that you put the cover up to? Because What If? comics don't count.
his having made the Heroes Reborn Earth into his personal fiefdom
So Doom only wins when you remove 90% of the opposition. Right.
having survived shots from Thanos using the power gem
So Doom only survives when Thanos purposefully fights with one arm tied behind his back. Gotcha.
having caught Captain America's shield mid throw
Thanos destroyed Captain America's shield.
having killed a lion when unarmed and unarmored
Thanos killed a universe while tied to a slab.
having managed to put one over on Mephisto himself
Thanos doesn't need to put one over on Mephisto, because Mephisto's no threat to him.
having immunized himself to Magneto's powers
Thanos doesn't need to fear Magneto.
having built a time machine
Thanos has time travel devices, too.
having built devices that let him move around in frozen time, having mastered a technique that lets him switch bodies with targets at will, being considered a legitimate candidate for Sorceror Supreme of the Earth dimension, having succeeded in developing a cure for the Thing's condition, having taken mental attacks from the Overmind and a Purple Man who had enslaved the entire planet, having thrown down in personal combat with Adam Warlock and the Magus...
And you really think all that is a comparison to what Thanos has accomplished? So he was a
candidate for Sorcerer Supreme? Ah, so he never actually got the job. Thanos, on the other hand, became the universe. He was all of existence. That's far greater than Sorcerer Supreme and he wasn't simply offered the job, he became the universe.
Doom fought Warlock and Magus? Thanos engineered the defeat of Magus and the Universal Church of Truth, as well as the death of Adam Warlock. He's the Avatar of Death and is the Magus's counterpart in the universe. And the rest of that is useless trivia. The fact of the matter is this:
Thanos is vastly more powerful on a personal level.
He has access to much greater and more advanced technology.
He's far more intelligent than Doom.
He's achieved far more than Doom.
He's a universal threat whereas Doom is a planetary threat.
In every way Thanos is the greater villain.