Galactus is more a force of nature than a bad guy, he isn't acting out of malicious intent.
Doctor Doom is definately the greatest supervillain of all time. He's a very human character (a guy in a suit) but at the same time to powerful, and believably so. In terms of oinkkicking contests I'd go with Doom, he always has the extra trick up his sleeve - I mean he even stole the Beyonder's powers and became a God!
The trouble with Doom is that he's too powerful. I prefer my villains up close and visceral. Hard to top some of the classic Spidey villains like Goblin or Vulture, in my book.
But then again there's always Magneto, the most rational of all villains... oh it's so hard to choose!