My votes. Keep in mind that I started playing in 1991, but never played through most of these and didn't read them (in PDF, generally) until the 2000s. So my vote tends to be more "do these still hold up" than nostalgia.
B4 The Lost City: I remember reading through the B-series modules and finding that this was the only one that really tripped my trigger, other than Night's Dark Terror (which felt like a proto-Ravenloft Setting module - a good thing, not a bad thing!) The extended, inverted dungeon... followed by the strange underground city... yes, now things were getting interesting!
L1 The Secret of Bone Hill: B2 bored me a little - it was quite clearly an introductory module, and it wasn't MY introductory module (those were Bargle's Dungeon from the Red Box, and Zanzer Tem's Dungeon from the Black Box). But L1... this was where it was at. A sandbox. A huge rumor table, a fully fleshed-out town, a mysterious keep with an interesting mix of challenges, side quests against monster tribes, wandering mercenaries, a temple devoted to gambling, even a burnt-out old guard house full of giant rats... this was much more interesting and less linear than B2! And while T1 might be superior overall, T1 always (coming in when I did) just seemed like the first part of Temple of Elemental Evil... not something that stood on it's own (even though it does and did for quite some time).
S2 White Plume Mountain: The original monty haul funhouse dungeon. What more is there to be said? It's like S1, only your friends will have a good time and won't punch you in the face after the whole party decides to climb into the demon's mouth at the end of the opening hallway. (One can see the difference between S1 - S1 is greater in it's reputation and what it's inspired - much better adventures like The Mud Sorcerer's Tomb, Return to the Tomb of Horrors and Tomb of Annihilation - whereas S2 is so perennially good that it's been updated for every edition with relatively few changes.)
T1 Village of Hommlett: Despite what I said above - it's still probably the best model of an introductory sandbox adventure. Unlike today's epic railroads and "adventure paths", you get the impression that, barring the much-belated T2-T4, you can go anywhere from here!
X2 Castle Amber: Not a tomb. Not a dungeon. Not a wilderness. A mansion full of mad/crazy/zany wizards. This is another one that can still throw players for a loop decades later. I've been toying with trying to launch a side career as a game designer/module writer - and if I do, my first module will likely be a homage to this one.