So the debate is already over.
Nobody really chimed in, which is so disappointing that I won't even comment on it beyond this, but I just finished reading Straczynski's run on Thor, which picks up with Kieron Gillen's run, and in case you didn't already know this....Doctor Doom is the greatest villain ever created. It actually makes sense too, because you kind of have to be bad-ass when you're created to go one on four with The Fantastic Four (who as a group are underrated these days.)
In Stracz's Thor run, which is solid by the way and actually spend more of its time focusing on his role amongst the Asgardians rather than his impact on the super-hero community (which I liked), Doom aligns himself with Loki in a major stratagem designed to impart him with power capable of rivaling Odin. It is bold. It is audacious. It has a certain arrogance that could only come from Victor Von Doom (who for those of you in the know is almost half as cool as Straud Von Chaos.) Seeing Doom make a play for divine power in the form of a super-charged version of the Destroyer armor was pretty cool. All in all it brings Doom back into focus around Marvel as THE bad guy.
That isn't where it stops though. Doom also took aim at Wakanda in the recent story arcs of Black Panther, showing a political agenda that suggests that he is preparing for a massive military move to conquer the technologically advanced nation. With a limited series coming soon it would appear that he is going to succeed, and that will change the power dynamic of Marvel for some time to come.
Still not convinced? His role in Dark Reign shouldn't be ignored. Nor the fact that twenty something years ago he stole The Beyonder's powers. His hubris alone should earn him a spot at the top of the argument.
Who could stand against him? Luthor? I like Luthor, but Doom is equally as smart, equally as cunning, but he wears waaaaay better armor and he's a dangerous mage. Luthor gets his ass handed to him. Ra's Al Ghul? I give that guy more credit than most people do. He has built an empire of power that has spanned for centuries and only a certain detective has ever been in a position to thwart him. He's ruthless, and he isn't above using magic and science to further his agendas. His cunning is easily Doom's equal, and his adversary may actually be more dangerous that super-genius Reed Richards. So why give the nod to Doom? Two things. Battle armor. Doom has the corner on the market. Also, Doom rules a nation, and wields the authority with open disdain for his opponents. Ra's moves behind the scenes, which makes him more lethal and scary, but not necessarily more dangerous. Doom brings world law into play every now and then, and its an element that few people consider when they think about how dangerous the bastard really is.
Who else could I consider?
Spidey's villains suck balls. Magneto might not even be a villain (if they would just make him STAY a racist prick, he'd be in the running.) The Red Skull? Please! Loki? There's something about proclaiming yourself the God of Lies & Mischief that takes the sting out of being a villain. Sinestro? Piss off. Zoom? Inverting your hero doesn't make a great villain.
No. I think it's Doom.
In a landslide.