Propellors.
Generally speaking, they are quite efficient at what they do, transform mechanical energy to useful thrust, to propel the aeroplane.
I can't speak about newer designs, but I do have numbers for one older type, the DC-6B.
Propeller efficiency...86%, give or take.
Surprised?
I'm not.
Hamilton Standard put a lot of hours/thought into propellor design, and the 45E60 blade is very efficient.