Aside from cockroaches and other insects, which species (of all kinds) are 'immune' to ionizing radiation?
According to Paul Stamets, Gomphidius glutinosus is especially well suited to collecting Cesium-137:


  G. glutinosus has been reported to absorb – via the mycelium – and
  concentrate radioactive Cesium 137 more than 10,000-fold over ambient
  background levels.


That article and Stamets' book Mycelium Running have more details on other species.