I came across a post a while back over at Grognardia regarding the top 30 D&D modules/adventures of all time. I've since then been struggling with my own list, but in the end, think it turned out pretty well. Check out my list, and then feel free to discuss, add your list, or reminisce about these picks!
Zachary's Top 10 List of D&D Modules
10. White Plume Mountain (S2): James over at Grognardia said this was perhaps a bit too artificial a setup, but I've never had that problem with it in that sense. Perhaps just a little on the short side, but great puzzles to figure out.
9. Dwellers of the Forbidden City (I1): A nice, hefty virtual campaign unto itself.
8. Dungeonland (EX1): I don't hear this one bandied about much, but this is a great Gygaxian dungeon, if a little unreasonably for higher levels.
7. Isle of Dread (X1): Pulpy fun. How fondly I still remember the maps!
6. Bottle City (Pied Piper Publishing): An enjoyable piece of history from the original Lake Geneva campaign.
5. The Village of Hommlet (T1): I love Temple of Elemental Evil, but Hommlet remains my favorite part. Likely one of the best introductions to D&D one could ask for (see my #1 pick for my other choice).
4. Against The Giants--Liberation of Geoff: I'm including the Silver Anniversary edition of this (yes, I know its 3 modules in 1), because that's the one I primarily played. This is a monster module collection, one that can give you months of adventures. This is a standard for any Greyhawk campaign I run.
3. Tomb of Horrors (S1): Your players will consider this cruel and unusual punishment. Then they'll come back asking for more.
2. Expedition to the Barrier Peaks (S3): Hardest. Damned. Module. Ever. This is a damned abattoir, completely with gonzo sci-fi weaponry.
1. Keep on the Borderlands (B2): A wonderful starter, and still likely the first module I'll run with my kids. As close to perfect as a D&D module can be. A no-brainer at the #1 spot.
Honorable Mention: Palace of the Silver Princess (B3): A very well-written beginner's module, this could on any given day squeeze into my top 10. There are a good half-dozen more that would warrant serious consideration, as well.
My worst? Easily Castle Greyhawk (WG7). Some jokes just aren't funny. And so it is with Castle Greyhawk. Even some the really cruddy modules out there, like The Doomgrinder or Gargoyle, have a leg up on CG.
I'd love to hear everyone else's lists!