My list of classic modules:
Basic D&D: B2 - The Keep on the Borderlands
* designed as a fun module, not as one needing much game logic, etc. I've run this in my new 3E campaign. It led onto...
1E AD&D: I3, I4, I5: Desert of Desolation
* The clarity with which these modules were written hasn't been surpassed. NPCs have personality and knowledge that means roleplaying with them is useful. Traps & Tricks are inventive and fiendish. Their only flaw is in how you get the PCs into them. (In my case, the passage to the Caves of the Unknown in the Chaos Shrine in B2 led to a magic portal - and the Desert!)
S1: Tomb of Horrors
* Very early module, showing what
not to do in design, but fun for the DM (if not for the players).
GDQ: Against the Giants, Descent into the Depths, Queen of the Demonweb Pits.
* Although more of an outline than a fully detailed adventure (particularly in D3), this showed the possibilities of an epic campaign. It also showed how successive mysteries could be layered on top of each other to provide interest and wonder throughout the series. Alas, Q1 is not anything near as good as it should be, and doesn't mesh well with the rest of the adventure.
2E AD&D: I never played many of the modules of this series, but I do agree that Feast of Goblyns is a fascinating adventure. (Is
Dead Gods a classic? Alas, I've never seen this, but heard much good about it).
3E D&D: The Sunless Citadel
* I just like it, though I've never DMed its entirity.
Return to the Temple of Elemental Evil
* Big, epic and a masterful weaving together of threads from scattered Greyhawk products. (For those who criticise Cook for using the EEG in a way not intended by Gygax... well, Gygax had his chance to lay it out years and years ago).
Cheers!