I would have to say,

- Tie Fighter (DOS game with a great twist on the Star Wars plot)

- Full Throttle, Sam & Max and Day of the Tentacle (probably some of the best DOS adventures ever made)

- Half Life

A DOS title, wich was fantastic:

"Terra Nova - Strike Force Centauri", from Looking Glass

http://www.the-underdogs.info/game.php?gameid=1142

PS: And Tie Fighter is also my all time favorite game!