My list is below.
1. Kingpin: He was behind almost every major event or was a big part of it.
2. Spider Carnage: One bad mofo.
3. Green Goblin: This guy could have been number one if he had been used more.
4. Dr. Octopus: He'll always be up there with the big boys.
5. Venom: Now here's a tough villain.
6. Hobgoblin: Sort of like Spidey's arch enemy next to the Kingpin.
7. Carnage: A second Venom.
8. Lizard: The monster of Spidey's foes.
9. Mysterio: He was tough to beat.
10. Dormammu: Yeah, he's Dr. Strange's greatest foe, but he was the base of Carnage, Venom returning, and Mary Jane almost dieing.
Any other lists?