Bacteria. Can live in all different environments that other animals can't.
Actually, perhaps the most resilient organisms known are tardigrades ("water bears"). Tardigrades can go into a hibernation mode - called the tun state - one that is more akin to "suspended animation" whereby it can survive temperatures from -253°C to 151°C, as well as exposure to x-rays, and vacuum conditions. When you place tardigrades in perfluorocarbon fluid (again while hibernating), at a pressure of 600 MPa, (that's almost 6,000 times atmospheric pressure at sea level) they emerge from the experience just fine . Even the bacterium Deinococcus radiodurans, the most radiation resistant organism known, only achieves this resistance under some conditions such as fast growth and in nutrient-rich medium.