For D&D/d20, I think my group had more fun playing Beyond the Veil than I've seen anyone have in a long time. Runners up would be Forge of Fury, and Ravenloft. I'm tempted to say Red Hand of Doom, but so far, I've only read it, not run it. I think it will take top spot once I have though.
Non D&D I can narrow down to:
Starfall for d6 Star Wars
and
Isle of the Dead for Chill
Non D&D I can narrow down to:
Starfall for d6 Star Wars
and
Isle of the Dead for Chill