Worst part?

[Roy595]

For me, it was the Storage and Archives (not the one with the water monster).

Storage... for obvious reasons. The memento that said the darkness felt strange and unnatural was particularly unnerving. It was the first encounter with the enemy and there really didn't seem to be many good hiding places; it was a pretty open area. I felt genuine dread and terror and it took me a good month to crawl through this area.

It may be kind of weird but the Archives was pretty bad for me. The complete absence of people (and their bodies!) and that awful, droning sound really bothered me... plus the ominous monster noises and I REALLY didn't want to go down that hall with the piano and windows. Something felt very, very wrong in the Archives.

For some reason the prison wasn't that bad for me... I was actually having a good time exploring and taking in the atmosphere (then I had a hallucination with a flashing grunt chasing me and that freaked me out pretty good lol).

Tl;dr: Storage and Archives.