If we're talking "great" adventures instead of "classic" (Classic, IMO, requires that a significant part of the players have actually played it and thus have shared memories of it), I have to mention Dragon's Crown for Dark Sun. It featured travel back and forth across the face of the world (well, an area about as large as Spain anyway), man-eating halflings, thri-kreen driven nuts, ancient defiled ruins, giants, sorcerer-kings, sneaking around, and lots of other stuff. Also a refreshing lack of huge dungeons for such a big adventure - instead, most of the adventure took place while travelling, with a couple of small dungeons here and there. Dragon's Crown is my favoritest adventure ever.