Nuclear weapons couldn't kill off even half of the rats on this planet.

Which is exactly why if we want to survive a nuclear war we need to start cross breeding with black rats and probably cockroaches.

Well that's an unpleasant mental image...

However, cockroaches actually aren't any better at surviving radiation than any other species, mythbusters tried it.

They can survive in harsher environments using less energy. Wasn't actually thinking about a rad-hard organism. Maybe throw in some flatworm, someone has to have a robust error-correcting code system built into their genetics.

Granted we would have to figure out a different neuro chemistry or physiology for having human level cognition on a much smaller energy budget.