Which monster scared you the most and why?

[Paddy™]

The Brute, almost entirely for the audio associated with it. Mainly these:

\Steam\steamapps\common\amnesia the dark descent\sounds\enemy\brute\enabled01.ogg
\Steam\steamapps\common\amnesia the dark descent\sounds\enemy\brute\amb_idle_whimp01.ogg
\Steam\steamapps\common\amnesia the dark descent\sounds\enemy\brute\notice_long02.ogg

The last one, triggered when the enemy notices you (as you may have guessed from the filename!), is fantastic because it sounds like the Brute itself is scared by your encounter, as if you're a confusing threat he just wants to get rid of, which makes him even more frightening. Kinda like Leatherface in The Texas Chainsaw Massacre in that respect.

Incidentally, this audio clip pretty much ruined my life:

\Steam\steamapps\common\amnesia the dark descent\sounds\03\03_amb.ogg

The audio engineers who worked on this game are geniuses.