Only one? That's tough. My top favorites would be X2: Castle Amber, X4: Master of the Desert Nomads, and X5: Temple of Death, and it's hard to choose one single favorite among them.
I think if I have to choose one, though, I'll go with X3: Curse of Xanathon. The first module I ever played in (so it's got nostalgic value), but I think it is also one of the most interesting, and certainly complex modules of a lot of the early classics. It is essentially a mystery story, and can be played on a multitude of levels- from the pure hack 'n slash (which is how we first played it), to a more complex roleplaying scenario with deep political and social interactions (which is how I DM'd it on more than one occasion). Love that module.