Headcrab zombies. they are messed up. a parasite latches onto you, carves your skull open, and siezes control of your body, whilst leaving some poor guy inside who's reduced to crying and wailing 'OH GOD HELP ME, HELP ME'. Now THAT is creepy. every time I see one of them lumbering towards me I imagine if I could recognise the tattered clothes of someone I knew on its tortured form and it makes me shudder.
For much the same reason, the worm parasites from system shock 2 are bad. even worse for how clearly they speak, but they don't look visually as ruined as headcrab zombies, poor wrecks that there is truly no hope for.