The Department of Energy's ENERGY STAR program does not have a classification for energy-efficient humidifiers as of 2015. The best humidifier currently on the market is the Honeywell HCM-350, according to home appliance review site The Sweethome.
The Honeywell HCM-350 is an evaporative humidifier that has a number of advantages over its competitors, including a tank formed from a single piece of plastic to prevent leaks. It is also designed so that no part that touches water has electronic components, so it is dishwasher safe. The Department of Energy has found that evaporative humidifiers such as the HCM-350 are more energy efficient than warm mist humidifiers, but may not be as efficient as ultrasonic humidifier models.