This is just an excuse for blatant opinionism.
For me it is System Shock 2. , even tho I am 99.9% a simulation fan.
SS2 oozes atmosphere.
You awake from sleep in a corporate space ship with no memory of recent events. The
atmosphere is extremely cold and lonely. Apart from your former crew mates who have
become zombie like and call them selves the "many" after some kind of parasite infected them,
you do not see another human through the whole game, apart from a few instances where
you glimpse some one through a window moments before they die.
As the plot play out, you find your self a pawn in a battle between a cold, callas, super
computer and the "many"; a vast biological infection.
The story and game are totally liner, but utterly immerse. The plot plays out in the
game play and through the last words of the other crew members that are stored in
audio logs or ghostly visions.
SS2 has plenty of faults. The end was clearly rushed to meet a deadline and it is tricky,
but not impossible to run it on XP or Vista.
However, what a great game! with a few mods, the graphics don't even feel to dated
and the gameplay is as tense and balanced as it was back in 1999.
*edit* Oh, and it's as scary as hell!
I gave up playing it the first 2 (3?) times because it made me jump so oftern.
What are everyone elses nominations?