If your absolute priority is the softest possible recoil then the lightest
possible bullet is your answer. HOWEVER, if you have to make a certain
power level and the choice is between a heavier bullet moving at
a certain minimum velocity to make the power level or a light bullet
moving at the minimum velocity to make the power level, I dont really
know which would produce the lightest recoil.
BUT, you can google recoil calculators and find a half dozen and
you can plug your weight and velocity numbers in to give you the
relative recoil result. Of course you would do this after you calculate
the velocities per each bullet weight to give you the power levels
you need.