Parameters to compare microcontrollers for low power consumption?
First, you have to decide power consumption in which state matters for you.

There are several cases:


sleep-mode, possibly with some peripherals left running (an RTC, a timer, a PWM output, etc.)
very low-speed clock mode used instead of sleep-mode, with some peripherals left running
normal (high) clock mode, with the desired peripherals


Checking the exact power consumption (including the needed peripherals, as they can make a big difference) from the data sheet, reading and evaluating both typical and maximum figures is the way to go. Using generic (\$\mu\$A/MHz) figures will probably get you only a general answer.