Many of us "Old School Gamers" look back fondly on the classic B2 for Basic D&D. Or G1-3. Maybe Tomb of Horrors.
Often out focus is on the TSR classics, but there are others out there (many from Judges Guild come to mind).
My favorite, possibly of all time - Death on the Reik for Warhammer Fantasy Roleplay 1st Edition.
25 some odd years I can't say for sure if the adventure was uber-awesome, but my memories of it are. It was a blast to GM and my players had a blast with it. Too bad the campaign came to an end when the party's wizard blew himself and most of the party up with a misplaced fireball. It wasn't so much the fireball itself, but the dozen or so flasks of oil that he carried to enhance his pyromaniac tendencies. Ah well, he did learn a lesson.
So, what your favorite and why?