Most energy-efficient Arduino-compatible MCU which supports SPI

I am in need to finding the most energy-efficient Arduino-compatible MCU which also supports SPI. It will run only few lines of code and it will use SPI and only 2 GPIO pins. Nothing more. Any hint?

Standalone 328P running from a battery. uC, xtal, couple 22pf and 0.1uF caps, 10K reset pullup resistor. |500x430 |500x240

@CrossRoads: thank you for the hint.

I also heard about ATTiny but I don't know how they compare with ATMega328P in terms of power consumption at the lowest level of power mode. I ask this because the system will work for just 1 second in every 30min, so the deep-sleep energy consumption can make a difference. Any hints?

The 328P is actually a “picopower” chip. It might not be quite as low-power as one of the ATtiny chips, but if you get rid of the other power-hungry components that an Arduino Uno usually puts “around” it, they are probably so close to “as good” that it isn’t worth dealing with the pain of trying to use something that is less well-supported by the IDE and libraries.