For 3e it's Red Hand of Doom, with its tons of classed humanoids, new monsters, Tiamat-as-a-goddess, battlemaps, miniature tie-ins, etc.
For 2e it's one of the metaplot module series for Forgotten Realms. They exemplified the "metaplot trend" that plagued much of 2e.
For 1e, it's Temple of Elemental Evil. The apex of 1e adventure design, with an extensive dungeon populated to the last room, Hommlett, the Moathouse, etc. So iconic, it warranted a revision in 3e.