Write-back cache is the most efficient when performance is paramount.
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.