Doom 3, because it's the only one I've played.
The was one point that I was so freaked out (graphics highest, lights off, etc.) that some random noise (not even a zombie or anything) made me literally jump in my seat (pretty high, too) and involuntarily fire. I was facing a wall, so had there BEEN a zombie, but involuntary finger-click would have done absolutely nothing except make a big hole in the wall...which it did. I was ashamed of how scared I was.
-rock