My first encounter with the vortigaunts in half life one.(i was 9,i guess)
so I was blazing on through the game, picking off enemy who could only melee with the pistol.
then, I reach a kitchen(If memory serves) and I see a scientist get zapped by something in a dark room. So I just go out to meet these guys and BOOM, across the room, hit by a bolt, life down, dead. Didn't even see them the first time, just heard their chant.
It took me a whole week to get through to that place, having drunk through god how many sodas from a vending machine to get my health up and more, and end up staying in the dark room for most of the time in fear of those bastards.
However, eventually got the nerve to charge 'em head on and pick 'em off one by one with a shotgun.
Another one? The tentacles from half life. what barney says still haunts me("They hear you..")
Oh, and second to the guy who said chainsaw guy and regenerators from RE4. Those bastards were cheap