Oblivion. I've been playing it on and off since 2006 and I find something new every time.
Non-TES I would have to say Mechwarrior 3. Who doesn't like giant robots that shoot at each other? Mech 3 did it the best because it had a realistic feel that Mech 4 never got.
The campaign was also excellent.
Its supposed to be a military operation gone wrong and it nailed that. A lot of the intel you receive in the mission briefings are out dated or just plain wrong. One of the last few missions your intel officer tells you that the enemy only has the capability to operate light mechs in this particular area (weigh only 20-40 tons). I prepared my weapon load-out accordingly only to run into a 90 ton assault mech almost immediately.
EDIT: added video link to the campaign intro
This post has been edited by King Coin: Mar 12 2011, 12:03 AM