D3 Vault of the Drow: An amazing underground sandbox with so much depth and colour packed into so few pages. As the culmination of all that came before it really delivered, and as the template for so much that came after, its place in history is assured.
I2 Tomb of the Lizard King: An ancient prophecy, a compelling plot, fascinating adversaries, evocative locations - this adventure has it all. It manages to deliver the authentic heroic fantasy experience from the start. The set pieces are incredible - the opening assassination attempt, the fight with the dragon Aulicus, the entry into the tomb, the tomb itself, the final show-down. Challenging and engrossing. A stunning achievement.
I3 Pharaoh: For pure dungeoneering, this adventure is hard to beat. A collection of highly imaginative locations with some wonderful tricks and traps, against a colourful backdrop. As the opening act of the Desert of Desolation trilogy, it shows great promise, but as a standalone, it's very satisfying in its own right. This is location-based, theme-appropriate dungeon crawling at its best.
U1 The Sinister Secret of Saltmarsh: There's so much going on in this simple adventure that it's hard to know where to start. The setup is wonderfully simple but the way that the implied story unfolds is fantastic. It's a plot-heavy adventure that disguises itself as a simple exploration of an abandoned house yet somehow never deprives the PCs of their agency. Plenty is left open for the DM to decide (the identity of the assassin or the dead body in the cellar, the secret of the alchemist, the relationship between the two magic-users, the characters of the smugglers, the town of Saltmarsh itself) while still remaining completely coherent as-is. The moral dilemmas are written into the structure without being thrust in players' faces (should we really just be murdering these people?) and set the scene for the follow up very well. This is the true classic of the era.
X1 The Isle of Dread: The definitive wilderness exploration. This adventure has more potential than can be unlocked in a single campaign. As the site for the existing material, it is already head and shoulders above most other location-based wilderness adventures. Tribesmen, zombies, dinosaurs, ancient horrors, pirates, jungles, cults - it has it all. As a site for further development, it's astounding. This is a place that, once you use it properly in a campaign, you'll be coming back to it over and over.