There must to be other requirements other than efficiency. Certain types of compressors are good at high pressure, low volume, others have inverse properties. Efficiency is not pressure per unit power input. Efficiency is pressure and flow rate per unit power input.

If you are looking for highest pressure, then I think the previous post is correct with piston type. The volume might be pretty low depending on how much power is available.

What good is a highly efficient compressor if there is no application? ;-)

Greg