I think Venom has the potential to be Spidey's arch enemy as he IS the anti-spider-man so to speak, he's everything spider-man would be if he hadn't been bought up the way he was, or had not learnt that valuable lesson of responsiblity. I just think that potential hasn't been hit on yet, and it's Venom's alter ego Eddie Brock which is the weakest part of the character, not Venom per-se. (ie. Imagine if someone like Norman Osborn, with similar motives, became Venom, the character himself would be much stronger).
But yes, GG is definately Spidey's ultimate nemesis. I'd even say character wise, he can be stronger than the Joker, because whereas Joker doesn't have a family, and Batman doesn't know him personally, he's just this insane person. With GG, everything he does effects everything around him, his son, his business, Spidey is best friends with his son, and Osborne wishes that Parker was his son. Especially when he finds out SPidey's identity the whole thing becomes alot more personal, all he has to do is one small thing and turn Pete's world upside down. So yeah, I think Green Goblin is very copable if wirtten correctly to be better than Batman's Joker.