For me AoM is very scary.
Because the atmosphere+ambient sound blend well,combined with poor illumination and flashlight.
Was so scared that i turn brightness in console 999+

Maybe the scariest moment is when you go down to basement in hospital,Screamer in Heaven map,and distorted faces in ending 1
Cry of fear main factor is the disturbing enemy and sound.
That first screamer..man that was fucking brutal,I even use that sound for my alarm and its too-goddamn-effective

The crawling section is equally scary,if you see a duct with many branch,expect some monster and ammp