HL2 has plenty of foes I hate:
Fast zombies: Taking these guys down BEFORE they hit me is damn near impossible. They come so fast, and their screams always throw me off for a split second. They certainly do a good job of throwing into chaos my defensive plans at the end of Lowlife.
Hunters: Why is it that I can never find objects to throw when they're around?
Regular and acid-spitting antlions: They never stop coming. When we take back Earth, the vorts can keep a few for their secretions, but the wild ones are to be EXTERMINATED.
Halo: Drones. Flying, fast-moving, and small. I hate them SO MUCH! Brutes are annoying, along with every subspecies of Flood, but I wish the developers had never included Drones in the game at all!