Water Bear: World's Toughest Animal

And they're only 1mm long. Tardigrade (Wikipedia)

Water bears are able to survive in extreme environments that would kill almost any other animal. They can survive temperatures close to absolute zero, temperatures as high as 151°C (303°F), 1,000 times more radiation than any animal, nearly a decade without water, and can also survive in a vacuum like that found in space.

Nature's Great Survivors: Water Bears