DOOM 3 - Scary enuff to discorage u frm playing it.Cud be the repetitive gameplay too.The sound design plays a big role in this game apart frm the detailed graphics.
FEAR - The name says it all.Scary shit.
Stalker - Strange, nobody mentioned it yet.Maybe there arnt many ppl who've played this buggy gem.That mindfreak guy in the underground scared the bejesus outta me.The effect was very unsettling and cool at the same time.
HL2 Ravenholm - Very creepy level indeed.Those leaping and hissing crawlies are sick.If i see them,i get the fcuk outta there(after dropping a grenade ofcourse)
AVP 2 - Remember playing this game years back at nite with all lights off and watching out for those mofo aliens n predators in Marine mode.
Farcry - Some levels with those badass mutagens and bigass rocker launcher wielding monsters.
RTCW - Those unholy things in the Crypt level along with those Nazi mutant creations are a pain.
Painkiller - Some levels are pretty foreboding.Remember a level with some demonic babies.