Quest for Glory saga rules! I also like Quest for Glory 1-2
I would have to call the original Test Drive my favorite DOS game of all time. I could never really get tired of it, with all the cars they give you and a very difficult mountain pass to navigate. Why can't anyone remake that game properly?
(As a side note, out of the other Test Drives, definitely get Test Drive Unlimited. Other than that and the original, the rest are severly disappointing. )