This is something that follows up my describing Bart Hamilton as a 'third-rate Green Goblin'. My original thinking was that obviously Norman is the best, and Harry's 2nd rate, thus Bart is third. Sure the terms don't have to be applied that way, and it ignores the fellow Norman used for a time as the equivalent of a Superman robot (See, I told you I, Norman Osborn, wasn't the Green Goblin, because there he goes flying by right there!), but now I'm not even sure I agree with that ordering. I think there's an argument to be made that Harry was the greatest Green Goblin. Let's look at their histories/accomplishments:
Norman - Well, he not only created the formula that gave him his powers, he also designed the suits and weapons, so all Green Goblins (and Hobgoblins) are cribbing from him to some extent. He had plans to become a criminal overlord, with the destruction of Spider-Man as the action that would give him the necessary credentials. He's the first villain to discover Peter's secret identity (back when that meant something, by gar!). He killed Gwen Stacy. He did. . . other stuff with Gwen Stacy. He orchestrated the entire Clone Saga. One could argue that given Seward Trainer's involvement in that, and his daughter's dissatisfaction with him, that Norman is responsible for the female Doc Ock. Maybe.
He framed Spider-Man for murder, got controlling interest in the Daily Bugle, and I think he had his own grandson abducted and pinned that on Spider-Man, I'm not sure on that one. He's had Aunt May kidnapped and buried alive. He's tried to make Peter into his "heir". His overbearing nature, combined with his frequent mental health problems, didn't do Harry Osborn any good. He put Flash Thompson in a coma. And he killed Ben Reilly. I'm sure there's more.
Harry - Harry doesn't have the level of carnage his father has, especially when it comes to destruction of life. But I think Spider-Man is in large part a story of emotion, and I figure it hurts Peter a lot more to try and fight his best friend, rather than his best friend's crazy father. The fact that Harry started trying to destroy Spider-Man was only made worse by the fact that at times he'd become the Green Goblin to be a hero, so he could rehabilitate his father's name. That he could have such good - if misguided - intentions, makes his fall back into dementia much worse for Spidey.
The thing about Harry is he worked subtly. Whereas Norman would actually threaten or attack Peter's loved ones, Harry would just make him think that's what he had planned. He'd swoop down and grab Mary Jane off the street and take her to the bridge where Gwen died. Oh no, he wouldn't! But it was just to tell MJ that what was going on was between Peter and him, and Harry wasn't going to hurt anyone else. He'd give Aunt May flowers, just to get Peter wondering whether there was a bomb or something hidden in them. He'd hire the Rhino to attack Peter Parker, and keep saying that Parker's 'secret was out', even though Rhino had no clue what he was talking about. He'd fly around in broad daylight, and stop right in front of Peter just to say "Hello", because it would drive Pete nuts. Peter had no idea when the attack was coming, and didn't really know how to handle it when it did come, because it was still Harry, his college roomie, best bud.
There was "The Child Within" where Harry got Peter with some hallucinogenic gas, that sent Peter into his subconscious, and tormented him with all the people he'd lost over the years. All his doubts and fears and feelings of loss, all at once. Harry knew all about this stuff because they were friends, Pete would share, which meant Harry knew just where to hit him.
And of course, Harry's masterstroke, getting the Chameleon to create artificial life forms that passed as Peter's long-lost parents, with Chameleon believing they were to uncover Spider-Man's identity (since all those photos Parker gets clearly show a connection). But I think the true goal was: a) for Peter to find out it was a lie, b) to see his "parents" killed, and c) to find out that the person behind it was his now dead best friend, a last little attack from beyond the grave. I mean that's just sick. Given that this is the event that pushed Spider-Man into the "dark" and "gritty" realm (when he started referring to himself as "The Spider" during internal monologues), I think you might have to give it to Harry.
Of course, two things might swing it back Norman's way. One, he's still alive (whether his resurrection was a good idea I'll let others debate, though if "Sins Past" is the best they can do with him, it was a bad idea). Two, Harry died saving Peter's life, moments after he'd gotten Mary Jane and little Normie to safety. Saving the person you swore to destroy, probably hurts your rep, but I think that might make it hurt Peter even more. At the very end, his best friend came back, got out from under all the crap that had pushed him to where he was, and saved his life. But he didn't get the chance to build on it, that was his last stand.
I don't know. I still have this desire to give the edge to Harry, but he's the Green Goblin I grew up with, so I'm probably biased. Your thoughts?