Wrong question!
IIRC it takes around 4ft lbs of energy to pierce a rabbit skull which almost all pellets are capable of at normal airgun ranges. Again, IIRC a 12ftlb pellet will still retain 4ftlb at around 75yds. Don't quote me on any of this, you only have to look at the tables in Chairgun to verify or not.
So the question should always be "What's the most accurate pellet for my rifle/barrel combination?". There are not many pellets killing rabbits if you hit em in the arse.