Game: Gears of War 2 (2008)

Delta Squad just didn't step into the crap this time. With this boss, they actually stepped into the monster itself. After ruining whole cities and devastating COGs with ease, the Riftworm makes for a formidable foe. This boss is actually a level itself with crushing organs, grinding guts and antibodies which will rip you apart. Add to it a heart that will drown you as you try to stop its beating and the Riftworm has earned its place on this list.