There are some people out there who are of the belief that the Green Goblin is, always has been, and should be Spider-Man's #1 Archenemy.
And...I'm sorry, but no. That honor belongs to Doctor Octavius here. Mr. Osborn has been the recipient of quite a bit of luck.
People will point to the fact that the Goblin found Parker's double identity. Of course, Doc Ock unmasked Parker the first time they met, he just didn't know Parker from Adam. Norman had the advantage of knowing Peter through his son Harry. So...luck, not skill.
Then there's the Gwen Stacy factor. Have you read that issue? Again, it was dumb luck and a serious miscalculation on Spider-Man's part that lead to Gwen's death. By comparison, Doc Ock had a higher bodycount until the return of Osborn in the 1990s and especially under Bendis, where his murderousness was amped up beyond expectations.
When it comes to general lethality, Norman made some nice toys, this is true. Doctor Octopus built goddamn indestructible arms and nuclear reactors, undersea bases, and dimensional portals. Doctor Octopus formed the Sinister six, not the Green Goblin. Now, granted the Goblin legacy (Harry Osborn, the Hobgoblin, etc.) has on balance, been as big (if not bigger) problem for Spidey over the years as Otto Octavious. However, if we are talking sheer level of threat, Osborn's a step above Electro, sure, but up until a few years ago he couldn't even measure up to the threat level of the Kingpin. Doc Ock was a world beater, pure and simple.