There was an interesting question on a podcast: How would you determine the ranks of Spider-Man villains? Who would be A-list? Who would be B-list? C-list? And so on?
A complicating factor is that there are some villains who are interesting because they're not very good at it.
A criteria I'm considering is that A-list is basically villains with staying power who could support a TPB length story.
B-list would be newer big villains (Mister Negative) as well as some of the classic guys who you can't really imagine being the main bad guy in a five part story. You could also make a case that derivative villains are B-list at best, so that Carnage and Morlun would be in this category.