The only thing that comes to mind that had anything close to the the cultural impact of Star Wars in '77 is the original Matrix. I'm not sure I've seen anything since then with such widespread collective resonance, but I'd have to agree that Dark Knight and Avatar have probably come the closest.

I doubt Star Wars VII has much chance of recapturing that kind of runaway success, but I'm trying to stay optimistic that we'll at least end up with something as fun and well made as Pirates of the Caribbean or Avengers.