Well, I'm not sure what qualifies as a "classic adventure," but here are my choices:
1. Forbidden City (from I1, Dwellers of the Forbidden City) - It introduced the Yuan-ti, one of my favorite villain races.
2. Castle Amber (From X2 - Castle Amber) - More zany, wacky, deadly fun cannot be found this side of Glantri.
3. The Demonweb (From Q1 - Queen of the Demonweb Pits) - A launching point for dozens of potential campaigns, each one an inspiration in its own right.
Honorable mention goes to: The City of Brass. This is featured in a boxed set from White Wolf and has several adventures set within the city. It's also the most bad-@ss city ever to grace the eternal flames of the Elemental Plane of Fire.