Limiting the list to an AD&D top 3?
1. The second Ravenloft Campaign Setting (red box, post-Grand Conjunction). My first in-depth introduction to the setting.
2. Council of Wyrms. Playing dragons is cool. (Or would have been, had I actually managed it.)
3. Spelljammer: AD&D Adventures in Space. Just a fun setting, and I liked the weird options and metaphysics.
Runners-up would be Masque of the Red Death, the Planescape Campaign Setting, Planes of Chaos, and Planes of Law. (I never did manage to get Planes of Conflict, but I'm sure it would have been a top one if I had.)