The original Thief: The Dark Project.
The fear came not from the graphics (even for their time they were merely functional). The fear came from a combination of oppressive and ominous atmosphere punctuated by lurking horrific creatures, all combined with a sound engine that still has no equal today. The original Thief is a masterclass effort in creating horrific dread within the player that just keeps growing thicker. There's no jump-out-of-the-closet-boo! scares here. You often know well ahead of time that an undead ghoul or insane ghost is in your vicinity, you're just not sure exactly where they are. You just hear their scuffling, moaning, and frantic mumbling all around you, while you bumble through dark forgotten passageways doing your best not to attract their dire attention. Attracting their attention means you're going to have a bad, bad time. This is the only game I've ever played at night where sometimes I had to just stop and wait until daylight hours.
And yes, I've visited Thief 3's infamous Shalebridge Cradle, and that level (while awesome) does not hold a candle's dimming flicker to the majesty of the original Thief's Haunted Cathedral. There's a plaque on the door of the cathedral that reads:
Warning:
Great evil resides in this place, and it is no longer fit for men.
The doors are sealed to protect us from that which lies within.
Do not remain here.
A fitting description for the entire game, truly.
