I was thinking this would be a serious talk on Marvel villains. But after reading 8 pages and no one mentioning Leap Frog I can see how wrong I was.
If I had to do a list I would do.
Doctor Doom
Magneto
Loki
Thanos
Ultron
I was thinking Green Goblin but up until the last decade or so he has been stuck in Spider Man's world. I did my list based on villains who have threatened the world and other heroes as a whole.