1 - Thanos - as powerful as Thor, as smart as Mr Fantastic. A technological, cosmic, psychic, physical and magical threat with a nihilist philosophy. The perfect villian.
2 - Loki - the god of mischief is oh so charismatic, a very powerful sorceror and is the nemesis of the most powerful of heroes. This is the guy who caused Ragnarok FCS!
3 - Mister Sinister - he makes morbid look artistic. Forget the psychotic killers, THIS guy creeps the hell out of me. In his hand you'll suffer many fates worse than death.
4 - Magneto - the villian that's relatable. We all wish we could sometimes turn to people who discriminate us and reverse their bloodflow.
5 - Apocalypse - Magneto may be the most recurring X-Villian, but when this guy shows up he's such a threat that Magneto joins the X-Men to try and stop him.
6 - Exodus - that's the guy that at peeks has fought all the Avengers and X-Men together and won. Plus he has all the coolest powers: telepathy, telekinesis, flight, immortality and teleportation.
7 - Green Goblin - yeah baby, he is crazy
8 - Mephisto - the coolest, meanest, smartest demon of the Marvel Universe. He rarely ever needs to fight anyone, cause he's so cool he's above that. "Let's make a deal".
9 - Enchantress - the ultimate seductress who bends mens wills with a kiss. This social alpinist is willing to do anything to achieve her goal, stealing Thor's heart. Hell may hath no fury like a woman scorned, but what about a goddess scorned?
10 - Baron Zemo - any of the Zemos. People always think of the Red Skull as Cap's nemesis, but this is the guy that "killed" Bucky, founded the Masters of Evil and founded the Thunderbolts and he's skilled enough to hold his own against Cap in a physical combat. To me, this is the Anti-Cap.
PS: I love Galactus, but don't think of him as a villian, but a force of nature.