L4D2 - Jockeys. They crouch, so they're hard to shoot, they leap like hunters, they ass-rape you in the face while beating the shit out of you, and then they send you off to die from a witch or fire.
Also, Tanks. Smack you once, you try to get up, and they're right there to smack you into the wall again, and keep doing the same thing over and over again until you're incapped, then they stomp all over you and ass-rape you until you die.
And the common infected. When you're trying to be fast and get the hell out of somewhere, a dozen show up and smack you around, slowing you to a crawl, and in some cases where they surround you, prevent you from walking at all. Just go stumble around somewhere else and let the Special Infected do the real work...
Modern Warfare 2 - Easy to kill with explosives, my ass. I shot one in the FACE WITH AN RPG AND IT LIVED. FUCK YOU, INFINITY WARD!
Dark Sector - The soldiers. They come out of nowhere in swarms and shoot you up, and it only takes a few shots to kill you.
Turok - EVERY FUCKING THING.
Pokemon - FUCK YOU ZUBATS AND TENTACOOL!
Fallout 3 - Giant Scorpions, and not the dog-sized ones that you fight around the beginning of the game, I'm talking about the gigantic car-sized ones that could eat a DeathClaw with a side of Yau Gai fries.
*deep breath*