Whilst HL & HL2 have their spooky moments, and the Resident Evil series of games have the odd "made-you-jump" moment, nothing, and I mean nothing comes close to the terror inducing System Shock 2. The entire game was built around making you scared, all the time!!!. Not by making you jump, not by the odd freaky moment. But by the relentless psychological pressure, by the fact the enemies respawned and sought you out, and by the brilliant use of sound.

When someone says a game is scary, the gold standard to which it should be compared is SS2. I'm not ashamed to say I was too scared to play it. So scared I uninstalled it after quitting the game in terror.