The only survivors of a nuclear fallout will be cockroaches and salesmen.
Cockroaches aren't spectacularly resistant to ionising radiation, although they are abour 10 times as resistant as humans.
The big winner is Conan the Bacterium, Deinococcus radiodurans. You need serious nukage to kill that. 1.5 Megarads won't kill it all. You can also dry it out, soak it in acid, freeze it in space...and it still lives. If any one thing is left alive, that's it.