For me it would be:
Fallout:
For choices and consequences, and being able to tune your character just the way you want him. For being able to get through a significant number of the challenges by talking. (As opposed to having to kill everyone) For being able to explore a post apocalyptic world fraught with danger, and giving us a memorable villain in "The Master". The Master really thought he was doing the world a favor by turinig them all into mutants. "New World Order" indeed!
System Shock 2:
For a successful blend of science fiction, suspense, and role playing. For throwing in some ghosts and screaming psychotic monkeys to keep you on your toes. In rememberance of Shodan, one of the greatest RPG villains ever. (Second only to "The Master" from Fallout)
Half Life 1.
Gave me a great sense of immersion, starting with the train ride into the Black Mesa facility. and continuing through many harrowing levels. It threw at us the best programmed enemy AI until that date. (The Marines) As well as some true fear of the unknown. While technically a first person shooter, the immersion gave you that feeling of survival horror,where you could easily identify with Gordon Freeman, who just wanted to go home. As an FPS, it gave you more RPG vibes than many other true RPGs of the time.
Respectfully
Falloutfan.