The superlative of efficient is "most efficient."
Spur Gear is the most efficient
More efficient and most efficient are the comparative and superlative forms of "efficient".
A screw is the most efficient.
most cost-efficient
LEDs are the most energy-efficient lights as of 2016.
more efficient, most efficient
most cost-efficient
Many SUV's are not fuel efficient.
The most efficient is a geo-thermal heat pump, if properly installed.
The Friedrich SS10L10* is the most energy efficient air conditioner with a rating of 12.
Memory is far and away the most efficient upgrade for any laptop.