Outlast really did have few tense and terrifying scenes. My personal highlight is the sewer section, when you are chest deep in the murky waters, and you don't see a darn thing in front of you, even with the night vision mode on, and of course something is stalking you in there. I was so immersed that I even spent almost ten minutes just standing in the corner and just listening, waiting.
Another great part when you find yourself outside again, and it is pitch black with occasional lightning bolt shining the way, I felt pretty much the same as in the sewers, these are the moments the game really shines.
However I really disliked that they used the same bloody enemy for each of these sections, that brute, although intimidating at first, really started to annoy me at that point.