Callisto 

 
Callisto is the most heavily cratered object in the solar system -- craters upon craters.

No magnetic field, hence no interior core. Lowest density of the Galilean satellites, hence thought to have the highest ice content.

Basically thought to be an undifferentiated ball of rock and ice.





Voyager found signs of extreme impacts, like the Valhalla basin:
 
 

And check out this chain of craters -- how might this have happened?