Actually, the OP asked about the highest energy per unit weight OR VOLUME.
In terms of producing compact but high power cartridges, the energy of SETTLED propellant granules per unit volume has a lot to do with how much performance can be attained. The proper term for that powder parameter is "bulk density."
A powder with very small rounded grains tends to pack the tightest (highest bulk density), but that also produces a lot of surface area per unit weight, so such powders tend to be "fast burning." So, tight-packing ball powders tend to require a lot of surface deterent coatings to produce slow-burning characteristics.
The various powder parameters are not physically independent of each other. The feasible manufacturing processes affect many parmeters with each tweek. So, what we get is (at best) the practical maximum with the technology available at the time of manufacture.
If the various theoretical powder properties could be made independently variable in some future manufacturing process, then there could be huge gains in cartridge performance, because only about 30% of the energy in the propellant charge ends-up as kenetic energy in the bullet at the muzzle with the high-pressure cartridges in use today.