"But by one hero alone? with no help from others?"
I doubt it.
Forget Doom's powers, gadgets and intellect for a second.
Doom is a NORMAL MAN. Considering he is just human, the fact that he be as freaking awesome as he is...well it's incredible. He is just the total package.
People say "without prep time" he can be beaten. As Victor Von Doom said, he was born ready. He doesn't walk around in his underwear, is always in his suit which as Victor pointed out, contains more gadgets than a lesbian orgy.
If Supes flew in from Metropolis, Doom would have a kryptonite shield around his castle.
-AC
These posts are a bit silly to be honest, because even the likes of Spider-Man, Cap America or even Daredevil could take down Doom, if the writers see fit, and the fact that Thanos could beat him on paper would mean jack s**t. Stats mean jack in the comic book universe, because if comic fights were down to stat comparring, then it would be very predictable and boring. So don't overlook the likes of your friendly neighbourhood superheroes in favour of those almighty ridiculous celestial fools.
Doom doesn't have to resort to amazingly complex battle plans though. We all saw what he did to Rage of the New Warriors. Made it so that if anything happens to him, everyone goes kaboom.
He has energy blasts and attack more powerful than most, so he could fight with the either of those two. Not forgetting his suit gives him strength to a very high class.
-AC
Known Superhuman Powers: While in his armor, Dr. Doom can lift (press) approximately 2 tons. He is one of the smartest men in the world and has an almost endless variety of weapons and equipment at his disposal. At the very least, Doom's armor contains a force field and concussion beams -- though more weapons may also be included. Dr. Doom also has the ability to switch minds with another person and he possesses a small amount of mystical knowledge as well.
his powers everyone.
Well, I don't know too much about comics or their characters, but I have to agree with the above people.
The small bit I do know about Doom points him to being a huge, powerful, near-unstoppable juggernaught. (No, not the one from X-Men.)
However, I think that, with the proper team going up against him, he'd be able to be taken down.
One on one, however, I'm pretty sure Doom will [i]always[i] come out on top.