Black Mesa Source, I absolutely love the game but the marines are absolute bullshit. They have laser eye accuracy and their SMGs do phenomenal amounts of damage at any range while the one you pick up off of them may as well be shooting dust at them in return. I know a lot of FPS games do this to some degree with the accuracy of their AI enemies but the Black Mesa Marines are the only ones that I've ever gotten properly angry at.
Also, Vermintide, the Skaven sometimes have broken animations that let them teleport across rooftops instead of jumping or vanish and reappear behind you. And on Cataclysm that one rat behind you will take off 1/4 of your health bar in a single hit. They also love to hit you through walls, ceilings, floors, etc which is compounded by trying to ascend a flight of stairs while blocking being somehow beyond the confines of the game engine, resulting in you getting stuck on random steps.
Also, Vermintide, the Skaven sometimes have broken animations that let them teleport across rooftops instead of jumping or vanish and reappear behind you. And on Cataclysm that one rat behind you will take off 1/4 of your health bar in a single hit. They also love to hit you through walls, ceilings, floors, etc which is compounded by trying to ascend a flight of stairs while blocking being somehow beyond the confines of the game engine, resulting in you getting stuck on random steps.