X2 - Castle Amber
It took the work of an author (Clark Ashton Smith) who was fairly popular in his time, but somewhat obscure these (er, those) days, and meshed it with D&D (I also like how they meshed it with the Known World/Mystara setting later on).
It was a very very odd adventure. It was something of a dungeon crawl, at least the first part (only set in a Castle), but it was a very weird one.
I also liked the art. It was one of the first modules (and only) that illustrated the same party of characters as they went through it.
I also really liked the job TSR did on the follow-up module, 15 years later. Though it's tied in a lot more to the Mystara/Known World setting, and the audio cd was a bit silly. But they did a good job of revisiting the same ground, without rewriting history or mocking it (something the Kenzer and Dungeon/Polyhedron people could learn from, I think, given how they like to rewrite old modules/settings in order to mock fans...)