Most energy-efficient low-end CPU
It is possible to benchmark an MCU for power consumption if you know what tasks you wish to perform. For example, an MSP430 might have a uA range current consumption, but it might require more instructions than an ARM M0 for the same task, and then, assuming their clocks are identical, you can estimate which one consumes less power. There are many other parameters which affect the solution's total power consumption.

Of course, if you have the hardware, you can compare the actual measurements.