Hands down, it is X-Wing from Lucas Arts. Can't remember it? That's because it was from, oh, fifteen years ago or so now?
Other favorites include -
(1) Mass Effect 1& 2
(2) Dragon Age 1& 2
(3) Knights of the Old Republic
(4) Alpha Centauri
(5) Total Annihilation (remember that one?)
(6) And I really liked Warcraft and Rift, though I didn't stick with Rift for the long haul.