Which of these classes of amplifiers has the highest efficiency?

Amplifier efficiency is a measure of how much DC input power is converted into useful RF output power instead of being wasted as heat. A Class C amplifier is highly efficient because its amplifying device conducts for less than 180 degrees (less than half) of the input signal cycle. Because it is turned completely off for most of the cycle, very little power is wasted. While Class C amplifiers can achieve efficiencies of 80% or more, this severe on/off switching highly distorts the input waveform. Therefore, they are generally only used for continuous wave (CW) or frequency modulation (FM) signals, not for linear applications like SSB or AM. Class A is the most linear but least efficient, while Class B and AB fall somewhere in between.