What organism most efficiently converts a given quantity of mass into heat?
A small hardy warm blooded omnivore that is easily tamed.  I suggest rats.

Small:  You don't want a lot of metabolic effort going into building bone and structure to support structure.
Hardy:  it needs to be survivable with little care.
Warm blooded:  You want to harvest energy from it.
Easily tamed:  The animal shouldn't try to escape its captivity.