The single biggest criteria that makes an adventure a "classic" to me is if I want to play it again, even though I know all the plot twists and surprises (just like a good movie or video game). I must have played I6 Ravenloft and the 2E House of Strahd dozens of times and I loved it every time. There's also the Temple of Elemental Evil, the Sunless Citadel, and the Forge of Fury (each of which I've played through multiple times and would likely do so again). The Bloodstone Pass series was great, so was Ruins of Undermountain and Rappan Athuk. I'd play those in a heartbeat, unfortunately, I don't know anyone willing to run them.