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...)
B4 - The Lost City
This is a very underated module, IMHO. Like the title implies, it details a lost city of a lost and dying race, which seems an awful lot like something out of Michael Moorcock.
It was very open ended - the hook was simply that the PCs were lost in a desert and found a lost underground city (I know, not the best of setups).
The goal of the adventure wasn't to sack the city, but explore it, and hopefully befriend (and unite) the inhabitants. But there were a variety of other things and goals.
What I also liked, was how the basic areas, where low-level parties could go, were described completely, but the down below areas, for when the party outgrew the levels of the adventure/series, were also described somewhat vaguely. Ultimately they could kill the false demon god of the place.