I'm gonna go with Mephisto. All of the others' paltry evil is small-time compared to his. He's not even interested in little things like killing or politics, either. His greatest desire is the corruption of pure souls. Not just killing a hero, but actually finding that tiny, dark part of them and exploiting it until it consumes them. That's a biblical sort of evil right there.
Besides, several of those dudes aren't even evil, really. Magneto's a misguided and megalomaniacal idealist, Loki's a petty child lashing out at those he perceives as his tormentors, Galactus is a fundamental part of the universe fulfilling a necessary function, Dr. Doom's another megalomaniac who just truly believes that the world would be better off with himself in control (and Reed Richards dead), etc.