When I was about 5 (over 20 years ago) we borrowed a computer, tape drive, monochrome monitor, it was awesome. The only thing it came with was a game called Transylvania, which, I have to say, was pretty well done for the time. Anyway, there's this bit where you have to run away from a werewolf chasing you, and we could never do it and always died when the damn thing popped up in front of you and ate you. I got so scared that I never went anywhere in the dark by myself
More recently.. Ravenholm (the first time through) was... unnerving. Also poison headcrabs and Fast Zombies... I hate the way they move at normal speed and then suddenly
leap at you unexpectedly.
Oh, and the first time I saw a Big Daddy, after it left I was scared I was going to bump into the thing after the cage opened. Actually, I'm surprised no one has mentioned Big Daddy yet... surely I'm not the only one who was a little bit nervous about them when they first make an appearance...