Shalebridge Cradle in Thief 3.
Yes, the top for me too.
There are other games with tense or horror levels, but while they can have more explicit horror level, they can't reach the Cradle for tension.
The Bonehoard in Thief 1 is scary, but less tense, you see immediately the dangers, and you can find some relatively safe places too.
The Halls of the Blind in Diablo 1 are scary and tense, but they are too small and the quest is quick and not too hard, darkness apart.
Scary levels abound in Tomb Raider 2, but they are more adrenalinic and almost splatter, but far less tense.
The feeling of insecurity was on point, especially when you had to pick locks lol There was another point that scared me to death - near the end of the game when the fat witch appeared and ordered her golems to kill Garrett in very creepy voice. I hate being frightened so I avoid horror games, but I had to make an exception for Thief 3.