Freespace 2 is my 3D space combat game of choice, and it's serious stuff. There's a real weight to the whole affair, exemplified by the wonderfully-voiced mission briefings, and actually piloting the various spacecraft through the missions is a pretty demanding business.
Starting out as a rookie pilot transferred to a new squadron, the player becomes embroiled in a complex plot that plays over a number of intense combat missions. Events unfold before your eyes as part of the action, and dialogue crackles over the radio throughout. It's one of the most immersive game experiences out there.
In its basic form the game is very attractive and still holds up today, but with it being open-source now there is a very active modding community, and the whole game can be upgraded visually and aurally. The results are amazing and set it comfortably alongside any contemporary space game. I've never played any other space combat game with such a believable feel, especially when the action really hots up and you're amid dozens of swarming ships. It's true white-knuckle stuff.