Actually, despite all my love for System Shock 2, I didn't find the big revelation particularly scary. It was the whole way the game was designed that really blew me away. It was how the ammo was always on the verge of running out; it was counting your bullets before going up against The Many; it was the weapons always being on the verge of breaking down; it was the way the logs made it seem like you were always about to find someone else alive but never actually managed to; it was both The Many and SHODAN continually harassing and egging you on; it was the way all this together made you feel so terribly alone; it was the way all those logs made you care for all the people dying, and even for the ones you had to kill. In fact, when thinking about it, the moment that really stayed in my memory was when you finally manage to catch up to Delacroix. :'(
Goddammit, System Shock 2 is truly awesome.
Edit: And the monkey shrieks, and the midwives' mechanical steps, and the noises the spiders made, and the robots' helpful voices, and positional audio, and the freaking ghosts. :ph34r: