Let me see, in appropriately the order I went "I love this module" while reading and re-reading through the list my picks go:
Q1 Queen of the Demonweb Pits - What, no love for Lolth? The extraplanar weirdness of the encounters, the innovative map of the Demonweb itself, fighting a Demon Queen inside her giant spider mecha palace. It's one of the greats.
S2 White Plume Mountain - A classic monster-filled puzzle dungeon that might not make much sense is a quintessential example of the genre.
S3 Expedition to the Barrier Peaks - A wonderful fusion of science fiction and fantasy, AD&D-style. There are some similar SF-D&D adventures in early D&D (i.e.
DA2 Temple of the Frog and
DA3 City of the Gods) that came out a few years after the cut-off point, but I don't like them as much! S3 has that gonzo gygaxian approach and a wide range of weird equipment, creatures and locations for the PCs to encounter and try to avoid dying.
X1 The Isle of Dread - Mainly because it's the urstone for early D&D hexcrawl adventures with the added bonus of dinosaurs. I prefer
WG6 Isle of the Ape, but that isn't an option.
A4 In the Dungeon of the Slave Lord - Being trapped in a cave system with nothing but a loincloth and your wits is a great challenge for players. I've only run it with the pregen characters since it didn't seem right arbitrarily dumping a party of established PCs into the adventure.
Hmm, a lot of my favourites are modules which stand out for having an unusual theme or location. The more generic D&D adventures may be fine examples of their craft or even better for broad playability, but just don't stand out for me.
Here's a few also-rans. I was seriously tempted to include
S4 Lost Caverns of Tsojcanth since it was a module I had fun running players through but as an adventure I mainly opened it up to read the New Monsters supplement (at least until they were reprinted in
Monster Manual II). Also tempted by
S1 Tomb of Horrors since it's such a well-known classic, but frankly it's the Gygax module I'd be the least likely to run - I prefer ones were the PCs have more chance of surviving - plus if I check Q1 and S1 to S4 it would look unimaginative!
Teased with the idea of voting
X2 Castle Amber (Chateau d’ Amberville) since it reads very well but I've never got around to running it
.
Was thinking of including a "Lost City" type adventure, probably
I1 Dwellers of the Forbidden City although
B4 The Lost City was tempting but
X1 The Isle of Dread ended up scratching my wilderness-exploration itch. Since it has dinosaurs as previously mentioned.