Modules I’ve run enough times to become treasured classics:
I6: Ravenloft - The hands-down winner. I’ve run this for more groups than any other module. Always successful, always memorable, and always different due to the module’s incredible design that randomizes the BBEG’s objectives. Also, GM-inspiration-gold in terms of character, setting, map, and theme design.
U1: The Sinister Secret of Saltmarsh – a not-too distant 2nd place winner, U1 is a terrific intro module for new or seasoned players. Currently getting its Pathfinder treatment for my kids’ 2nd group of PCs.
UK3: The Gauntlet – Assailing, then defending a castle/keep? What’s not to love? Another memorable adventure that was successful with many groups over the years.
UK4: When a Star Falls – (Can you tell I like the AD&D modules from the UK?) It’s a great adventure that blends wilderness travel, dungeon exploration, castle infiltration, & role-playing into a great self-contained story. I’m also a sucker for the Tower of Heavens and it’s group of sage-oracles. An incarnation of the Tower has been present in every campaign game world I’ve run for over 20 years (& coming soon to a Golarion campaign near me!)
G1: Steading of the Hill Giant Chief – Heavily modified for insertion into an ongoing Forgotten Realms campaign, this was the only module of the series that was utilized. However, it was a very memorable session of PCs having one of the toughest slugfests of the campaign. When they emerged victorious (battered & bloody with a high body count…) they felt that they had really earned the title of “heroes”. Made even tougher by adapting it to Rolemaster. You haven’t seen a player wince until you’ve had a hill giant roll a “D-critical” with their oversized club…
B1: The Lost City – The Gold-standard module of the BECMI era. As others have said, a seemingly standard dungeon crawl reveals itself to be so, so much more…
X1: The Isle of Dread – The (previous – the title has now been taken by Kingmaker, IMO) gold-standard for sandbox adventuring. Great thematic module and always a player favorite.
I4: Pharoah – Another themed-classic, this time taking on the pyramid-tomb-robbing genre. While I was much less enamored with the 2nd and 3rd parts of the series, this one’s a keeper.
Destiny of Kings – Notable for not being the traditional “dungeon module”, this one has a compelling story, knights & politics, and the opportunity to save a kingdom. Popular with multiple groups when I’ve run it, it was a transformative module for me as a GM steering me to include more intrigue and politics into my games than I ever had previously.
W1: Crucible of Freya – Not only did it illustrate and sell me on the idea of “3rd Edition Rules, 1st Edition Feel” but it also showcased modern module design where a lot of the classics of earlier editions were clearly mired in site-based design. An excellent blend of all the right fantasy RPG elements.
Crypt of the Everflame – A Keep on the Borderlands-style home-base : wilderness trek : dungeon. It’s a new classic in my book for several reasons. It was the 1st Pathfinder RPG adventure that I ran & it was the 2nd module I ran for my kids, so it’s immediately inscribed in the nostalgia archives. However, this like Necro’s entry above, is a modern entry-level module that has all the necessary components. The dungeon is a top-notch starter as well, with a decent spread of monsters combined with riddles and traps in glorious old-school style.
Honorable Mentions that I think are Classics but that I haven’t had a chance to run yet:
Rise of the Runelords #1: Burnt Offerings
Kingmaker #1: Stolen Lands
G6: The Gray Citadel
The Enemy Within: Shadows Over Bogenhafen