It was a little later than 'childhood' because I didn't really get into gaming until I was in college, but I would have to say Outlast was my foundation when it came to horror games. I had so much fun playing it over and over, and I still revisit to this day, even through I know it like the back of my hand.
Oddly enough, because I hold Outlast in such high regard, it's kinda of difficult for me to play walking sim-esque horror games that I truly enjoy because I have yet to find one that give me the same sense of satisfaction while playing it (the only exception being the first two Amnesia games).