... Gygax's design genius in Keep on the Borderlands.
Think about it ... so many groups, having so many different and varied experiences, but despite that having a common baseline for discussion of their experiences in the Caves of Chaos. I attribute that in part to the design of the module -- it's a sandboxy kind of design, so the story flows from the DM's set up, the players, and their characters' actions rather than any explicit goal outlined in the adventure -- as well as its style, which tends to be spare and allows the DM to fill in details in a way to fit the group.
The adventure's design with multiple types of challenges to interact with as well as a near-infinite number of ways to approach the Caves of Chaos ensures variability and make repeat play enjoyable. I'd like to see this adventure available with whatever the introductory 5E product is.
Well done, Gary. Your adventure is more than 30 years old and still works as well today many iterations of D&D later. We miss you.