6. Poison Headcrab (Half-Life 2)
These pesky buggers have the distinction of actually making me let out a high-pitched scream the first time I encountered them in Half-Life 2 (gimme a break, I was 16). Just when we'd gotten used to the movement and attack patterns of the regular headcrabs, Valve decided to scare the bajesus out of us once again by introducing an evolved form of headcrab. Black in appearance and scuttling around in a very unsettling way, their worst attribute however was their ability to lunge vast distances, and also employ the services of other creatures to throw them at you. If they successfully landed on the player, they would immediately knock your health down to 1%, made even more stressful by your HEV suit computer declaring that you're basically as good as dead. Though they're encountered quite a lot throughout the Ravenholm portion of the game in particular, they're one creature we never felt numbed to or bored of; they consistently left us - or me at least - close to huddling in a corner, repeatedly saying "Find a happy place".