I think that the DOS games I played as a kid -
Pharaoh's Tomb,
Duke Nukem,
Crystal Caves,
Monuments of Mars, and
Chip's Challenge - are still my favorites today (except
Crystal Caves: the difficulty/fun ratio is a little too high there, for me). A lot of that could be just nostalgia, but for whatever reason, I still say
Pharaoh's Tomb, with its wonderfully creepy four-color graphics, can beat out just about any other game.
Of the games I've discovered more recently, here on Abandonia, though, I'd say
LOOM is the best. Something about the low-palette graphics is just really cool, and the concept of using music to interact with objects in an adventure game is absolutely unique!