I don't think anything from the current edition can be considered "classic" until a new edition comes out.
That said, I think Rappan Athuk has the potential to be "classic", especially when the revised version comes out next year (that will add more adventure and clean up the minor issues with the trilogy).
Mayfair's Elves and Lich Lords were pretty wicked.
But seriously... (not that they weren't wicked, but I think we know what you were talking about):
The Ghost Machine (one half of "To Stand on Hallowed Ground")
Banewarrens
Rappan Athuk
Of Sound Mind (especially great if you have a "druid specialist player" in your group!)
I wouldn't agree with that. A d20 adventure certainly has the capacity to be exemplary and/or create reminiscent moments.
Dead is partly right in that they will never be classic in the sense that they will never be the shared experience that they once were. That's what happens when you don't have once source for all adventures.
Possibly the Witchfire modules could be added to the list as well. After all, they garnered Iron Kingdoms a "best setting" ENnie several years before the setting was actually released.