The Village of Hommlet. It is to AD&D what Keep on the Borderlands was to Basic. The start of so many adventurer's careers. I remember it being one of my first times playing D&D. We raided a peasant's hovel and I felt horribly guilty. This pulled me into the adventure emotionally and I felt the need to right wrongs and defend the village from the evil out there. Personally I always loved this version with the Dave Trampier cover over the later green cover with Jeff Dee art.
There were nefarious individuals, plotting, planning evil things in the 'innocent' little village. The nearby moathouse (yep, ruins, the best place for adventures) held unknown dangers and treasures. And the suggestion of a still greater evil unwakened - the mysterious Temple of Elemental Evil - it seemed like we waited forever to be published, and boy did it not dissapoint! The first of the "supermodules", packed with more entertainment for your dollars than any book or movie could deliver.
This is the highest ranking module on my list. It was hard to choose, there were so many great contenders. The honorable mentions are:
WG4 Forgotten Temple of Tharizdun, which has a special place in my heart. While most of the adventure was typical, it was the weird lower levels where Tharizdun was imprisoned that made this adventure so special. And the art was appropriately strange and unlike any other thing TSR published.
The drow series is so famous I hardly felt the need to mention it. N1 Against the Cult of the Reptile God had an incredible - nearly impossible - ending "boss" monster and the lead up to it was scrumptious. S4 Lost Caverns of Tsojcanth felt like Gary Gygax mercilessly unleashing his imagination, it was such a great adventure and it had the best sourcebook included. I could go on.
But as great as all these other adventures were the Village of Hommlet will always rule my imagination and memories as the best module I ever played and DM'd.