Well, for actually being played I would have to say T1-4, Temple of Elemental Evil. We probably spent around 1 year real time playing through this module, and the areas around it. We continued on to the Slavelord stuff, but it wasn't the same.
Of the stuff that I've read but never played, I would have to go with Feast of Goblyns (sp?), the first module for the Ravinloft campaign set. I've always, always wanted to run it, just due to the mood that it sets. Maybe if I ever find a group to game with again...
-Purple