Today's question is just for people who have actually bought and used adventures for use with tabletop roleplaying games: what is the single adventure you got the most use out of and why? In my own case, there's not a clear-cut answer, though I'd probably guess that one of the classic low-level D&D modules like In Search of the Unknown, The Keep on the Borderlands, or The Village of Hommlet probably gets the prize, because they're very easy to just drop into a new campaign as a starting point and flexible enough that I can change their details to suit whatever kind of campaign I want to run.
I'm interested in this question, since adventure modules were once a staple of the hobby. The product lines of games like D&D and Traveller back in the day consisted of a lot of adventures. Today, that seems less common (Pathfinder being a notable exception) and I'm curious as to why that might be. My hope is that, by finding out which adventures people have used repeatedly, I might get an inkling of why adventure modules ceased to be as common as they once were.