[judge2020]

Sure. "STM32L1 MCUs also feature the industry's lowest power consumption of 170 nA in low-power mode with SRAM retention. ". But I think we're looking for the 'most efficient' general purpose computer.

https://www.st.com/en/microcontrollers-microprocessors/stm32...

[genewitch]

Intel also made the quark which is a 486 that runs on a button cell battery, the same type that we'd call a cmos battery.