D3: Vault of the Drow. Possibly Gygax's masterpiece, we get drow for the first time with some amazing descriptions of pure evil mixed with cold beauty (take a visit to the local torture parlors, etc.). Set the standard for a generation.
Let this article sway you.
S1: Tomb of Horrors. Topping D&D's own list, I've run this in 2nd, 3rd, Pathfinder, and 5th edition. It's iconic and there's few gamers who don't know what it is. It broke all the norms, set the standard for a "thinking man's" dungeon, and took it up a notch for players who felt with all their gear and wiles they had conquered AD&D. Also made great use of visuals, a lost concept nowadays with word and page count.
T1: Village of Hommlet. Another Gygax gem, a model for introductory adventures leaving you wondering what secrets lie behind each door and cellar.
X1: Isle of Dread. First hexploration module. Set the standard for another generation. An ultimate sandbox of "here's your map, go forth" (and don't run into a T-Rex or King Kong).
C1: Hidden Shrine of Tamoachan. Superb dungeon writing with some of the most imaginative rooms, an inverse concept (drops you in the middle/bottom), and a mechanism to keep you moving (originally a tournament module). Plus, one of the first forays into something non-European, inspiring future products, along with excellent handouts.