It occurs to me, reading this, that most of the adventures I love would not be considered a "defining" D&D adventure. I mean, take a look at my three favourites:
Hidden Shrine of Tamoachan (Aztecs, poison gas, and a talking crayfish that thinks he's a god)
Expedition to the Barrier Peaks (robots, vegepygmies, and a crazy fitness instructor android)
Isle of Dread (dinosaurs, flying raccoon things, and zombie cults).
All are super cool - but none of them really seem "iconic" to me. I would not hand one of these adventures to a newbie and say "hey, this is what D&D is usually like". I would hand those adventures to newbies and say "hey, this is some of the weird stuff D&D is capable of doing".
Really, most of the modules I've played have been the opposite of "Defining D&D" - I buy weird modules because I want something different from the usual D&D adventure. But, to stay on topic, my most defining D&D module is...
REF 3 and REF 4: The Book of Lairs 1 & 2.
Reason? Each book contained dozens of mini-adventures centred around a single monster type. They were made to be expanded upon by the GM, but could be picked up and played as is. They ranged in levels, and were great for a pick-up-and-play session filler (sort of the 1e equivalent to the Dungeon Delve book, which could also be on this list). Way back in 6th grade, I ran an entire campaign based off those books, it seemed.