G'day Erik!
My classic modules would have to include:
DL1 - Dragons of Despair
An extremely inventive setting and storyline. Although the other modules in the series don't always work well, the first is a classic, especially for the map and setting of Xak Tsaroth
G1-3 - Against the Giants
It was the first, and that's about enough by itself. What makes it special is the layered revelations that appear throughout the series. If G1 had stood on its own - not that good - but it continued through to...
D1-2 - Descent into the Depths of the Earth
An extremely different adventuring area, far, far below the surface of the earth.
D3 - Vault of the Drow
Where all manner of revelations occur about the AD&D world. Although the drow are cruelly misused later one, this one is glorious.
Q1 - Queen of the Demonweb Pits
Somewhat out of place in the GDQ series for reasons I've written about elsewhere, this is a strange, strange place which really deserves classic status.
I3 - Pharoah
My all-time favourite module of all time. I don't think there's a better blend of epic plot, role-playing, problem-solving and adventuring. The formatting of the entire module is also gloriously clear.
I4 - Oasis of the White Palm
There's so much going on in this module - a gloriously rich tapestry. Great fun both to play and DM. NPCs are given enough detail for the DM to make them memorable, without too much being given as to make the DM refer to pages of notes.
I5 - Lost Tomb of Martek
Here only because it finishes the
Desert of Desolation series, it's okay, but not as great as the first two.
S1 - Tomb of Horrors
I like killing PCs. Legendary status because of its deadliness - and because really good players can complete it.
S2 - White Plume Mountain
A very classic plot - recover the stolen magic swords - and some real 'old-school' D&D encounters that require good playing skills to overcome.
I am not particularly fond of S3 or S4.
B2 - Keep on the Borderlands
Kick the door down, kill the monster, take its treasure. The best introductory module for any version of D&D. (The Sunless Citadel is second, IMO). You don't want every module like Keep, but it's a great way to start.
I6 - Ravenloft
Just because.
T1 - The Village of Hommlet
It's flawed, but it's got a lot of things going for it. T1-4 is not classic, IMO.
None of the following qualify as "classic" in my mind: WG4, WG5, WG6, I1, I2, A1-4, T1-4, U1-3.
Cheers!