Love reminiscing on the DOS days...many of my favourites have already been mentioned (Quest for Glory games, Eye of the Beholder...) but I have a soft-spot for some of the lesser known D&D games from the ahem..unpopular campaign settings....
Dark Sun: Shattered Lands
Al Quadim: Genie's Curse
Ravenloft: Stone Prophet
Apparently I have a thing for deserts?