And there's the M0+, which is the newest in the M0 family.
The M0+ has 2/3 the dynamic power consumption of the M0, some nice features of the M3 and M4, fewer Flash accesses (which lowers the power even more), and better debugging.
Basically, if you are looking for the ultimate in low power, choose an M0+.
The Atmel D21 on the Zero is an M0+, and Atmel has just released an L21, which is an even lower power M0+.
The M0+ has 2/3 the dynamic power consumption of the M0, some nice features of the M3 and M4, fewer Flash accesses (which lowers the power even more), and better debugging.
Basically, if you are looking for the ultimate in low power, choose an M0+.
The Atmel D21 on the Zero is an M0+, and Atmel has just released an L21, which is an even lower power M0+.