My list:
AD&D division:
C1 Hidden Shrine of Tomoachan
D1-2 Descent Into the Depths of the Earth
D3 Vault of the Drow
G1-2-3 Against the Giants
I1 Dwellers of the Forbidden City
L1 Secret of Bone Hill
S1 Tomb of Horrors
S3 Expedition to Barrier Peaks
S4 The Lost Caverns of Tsojcanth
T1 The Village of Hommlet
WG4 The Forgotten Temple of Tharizdun
D&D division
B1 In Search of the Unknown (The first 2 printings had a "how to use w/ AD&D section)
B2 Keep on the Borderland (The first printing said it could be used w/ AD&D "with minor modifications")
B4 The Lost City
X1 The Isle of Dread
X2 Castle Amber
My caveat is that anything after about mid-1982 is not a classic. It might be very, very, very good - near mandatory to have in an OOP D&D collection, but still not "classic" in the old school (A)D&D sense.
Someone mentioned "classic" 2e modules. The problem with naming classic adventures in 2e is two-fold...
1. During the 2e years TSR really concentrated on setting material instead of adventures. TSR also increased its AD&D output tremendously. In my opinion, the attention to gameworlds as opposed to adventures and the necessarily limited testing time really shows in the generally poor quality of adventures put out. Sure, there were some great 2e adventures, but there was never the run of consistant quality that 1e had from '78 to '82.
2. Most 2e adventures were set in a particular gameworld. They are not as universal as 1e adventures. Hence, a fan of, say, Planescape who wasn't interested in Ravenloft, will not necessarily have had contact with any Ravenloft adventures. Thus, as opposed to 1e, where there were adventures EVERYBODY played, there aren't as many 2e modules that pretty much everybody had contact with.
R.A.