My ESP32 microcontroller web server!
I use the ESP32 microcontroller for many IoT projects. Often I take advantage of its WiFi feature and use it as a webserver.
This device is under $5! Correct, you read that right. I host web pages in production on a $5 device! Just like my Raspberry Pis this is a very economical to use and only consumes half a watt of energy. Like the raspberry Pi, it requires no cooling equipment or external peripherals. In fact, it also doesn't have an SD card. The entire server runs out of its internal flash memory.
I'll argue that there are no more efficient servers out there. Given that a typical rack server uses about 350 watts, the ESP32 is 700 times more efficient.