Tanks in Left 4 Dead. Despite that there are many enemies that kill you in horrible ways, and many more that are just damn tough to kill, Tanks are in a league all of their own.
Tanks have this tendency to do things the hard way. Even when you're ready for them, knowing exactly where they're hiding, with a Molotov ready, an escape route planned, and your entire team ready with assault rifles arranged in a neat group with perfect line of sight... Then a Smoker grabs you, the Tank rushes out, and by the time your teammates down the Smoker the Tank already hurled a car at you... And it just goes downhill from there.