Fav...
Geez... that IS tough...
Before giving my final vote, here are my honorable mentions...
B10 - Night's Dark Terror - I converted this to a 3rd edition module, upped the baddie powers, and ran my PCs through it starting at 6th level. They were 10th level when we finished... but it took 6 months! Great module!
B7 - Rahasia - Very strong on story, long and happy on twists. Love the "symbiotic powers" of the witches and the fact that they possess innocents, forcing the party to kill instead of capture... especially good if the last of the witches possesses a party member w/o the knowledge of the others.
Basic Set Dungeon - You know, the one in the DMG where the PCs are entering the lair of Bargle the Infamous? Great for newbie DMs.
B1-9 - For the bare fact that it gave you grand tour of the Grand Duchy of Karameikos and showed you how to tie adventures together.
X10 - Red Arrow Black Shield - For sheer mass combat goodness, it doesn't get better than this at intermediate levels.
X8 - Drums on Fire Mountain - Sadly underrated. You have a well-developed villain and an interesting culture clash.
X1 - Isle of Dread - Similarly underrated for its cultural and role-playing aspects.
T1-4 - Temple of Elemental Evil - Slightly overrated IMO, but still the biggest baddest dungeon around.
DA1 - Adventures in Blackmoor - This adventure was 15 pages of backstory, 15 pages of NPCs complete with motivations, and about 18 pages of actual fighting/adventure. Still strikes me as the best way to do a module since the DM knows exactly who is doing what and why... including historical reasons.
This is my second-favorite. DA4, the Duchy of Ten, was good too, but DA2 and DA3 were a bit disappointing.
And the winner is...
N4 - Treasure Hunt - This is IMO the quintessential adventure for newbie players AND newbie GMs. I have used this as a jumping off point for multiple campaigns and it's always a blast... escape from pirates, get marooned on an island, fight the elements, meet a wacky old coot, converse with a vengeful goddess set on destroying the island, infiltrate a manor where goblins and orcs are fighting a turf war, sneak into the cellars/basement below the manor to find the hidden boat, betrayal by the old codger, and a twist at the end when you DO find the boat. *drools* Excellent stuff. Some PCs may not appreciate the "you're captured and lose your stuff" at the beginning of the adventure, but if you use it as, "this is how you started your careers" I have never had a problem. But it probably *is* unusable as anything but a campaign kickoff. That said, it's the best campaign kick-off I know of. Ever. And that's why I like it... you get your players attention and hold it the whole time, and by the time they're done, they have some experience, they have some stuff, and they have had a wonderful ride.
--The Sigil