Good old Headcrabs.
I like how they are not just some disease made in a lab like oh so many zombie style lifeforms, or a supposedly highly intelligent sentient parasitic lifeform that actually completely sucks balls like the Flood from Halo.
They're just trying to get by and propogate their species, a headcrab latches onto a guy, turns him zombie, he trudges up to the nearest dude who happens to be Gordon Freeman, gets shot and dies.
Its brilliant!
(WIth regards to the Flood, I know that the Flood beat up an ancient race so much that they were forced to commit a galatic exterminatus with an array of big hula hoops, but to me you do not get that feeling of being overcome or destroyed by the slimey buggers in game, at all, they just seem so weak and downright silly even. I think we've had enough of power armoured big silent dudes for now, if there are going to be future Halo games there needs to be a Halo prequel set in the Forerunner-Flood war, done right it would be bloody brilliant.)