The classic Expedition to the Barrier Peaks!
I love this module. Lasers and Magic items Whoo hoo! I think I have used this module in my games over the years more than any other module in my collection, well except for maybe Keep on the Borderlands, followed up very closely by In Search of the Unknown. But the twist is that I have NEVER used it for a straight up D&D game. I've used it in two Gamma World Campaigns, Rifts (the player Dragon Hatchling fighting the Froghemoth was priceless), Star Wars, Heroes Unlimited (almost had a TPK on that one!), Ninjas & Superspies, d20 Modern, and more recently Mutant Future ! Yet I just realized I never used it for a regular straight up D&D game. How wild is that. Just further credit to the genius of Mr. Gygax. I just may have to rectify not running this for "regular" D&D at some point. Course I need to track down a new copy, mine is ragged and worn out from such heavy use over the years.
One of my absolute favorite drawings by Jeff Dee!