Some of these aren't DOS-based, admittedly, but...
RPGs:
Planescape: Torment
Fallout 1 & 2
Jagged Alliance 2 (sort of a strategy game with lots of RPG-ish elements)
Adventure games:
LucasArts' whole adventure game catalog. Particularly:
Grim Fandango
Indiana Jones & the Fate of Atlantis
Monkey Island 1-3
The Dig
Also:
Beneath a Steel Sky (it's even freeware!)
Sanitarium
Heard lots of good things about the Tex Murphy games, though I haven't gotten to play any myself yet.
Finally, there's Star Control 2 (also freeware now), which is like a RPG, adventure game, strategy game, and space shooter rolled into one yummy package.
Peace & Luv, Liz