[Honda100]

Just in general, the automatics of old that use a traditional torque converter are the ones that are inefficient because they are constantly slipping. It's like swimming, you can expend a lot of energy, but because your arms are effectively slipping in the water, you are also wasting a lot of energy pushing the water around.

Manuals are simply solid shafts (oversimplified) and directly hooked to the wheels when the clutch is engaged.

CVTs are the same story except that they use steel bands (or rubber belts in small ones) that hook the transmission directly to the wheels. I don't know if CVTs now use a torque converter so I can't comment on the efficiencies of big ones, but I know my Honda Metropolitan uses a centrifugal clutch, which is a "non slipping" engine to trans engagement mechanism.

Double clutch automatics are much more efficient because they are essentially two manual transmissions in one, so they don't slip like traditional automatics.

Honestly though, anything that isn't a traditional automatic is going to be the most "efficient".