Well, what are the use cases of your system? That's what will drive this decision.
Here are some considerations:
If the system spends 99% of its time sleeping then the key parameter is not going to be max current draw. It is actually the sleep current.
I suggest looking into CoreMark when comparing between controllers. CoreMark has a power metric which is basically performance/mW. So if you wake up from sleep, how efficiently can you do what you need to do (perhaps log data) before going back to sleep. Unfortunately this list is pretty short right now, hopefully it will grow over time.
In a data logger / metering application a key thing to consider is how much power your memory is going to take (in the uC or external), because that will be a major consumer.
You should also consider various sleep modes. Does the controller have a sleep mode that supports internal ADC that can wake up the core and interrupt when a conversion is done if you need it to? Does it have an RTC that can run in sleep mode that can wake the controller up to poll for data periodically or at least maintain system time if you need it to? When the controller is in those sleep modes with select peripherals still operating what is the current draw? High end MSP430s like the 5 series support 5 sleep modes. The 1 series only has two I think. Do you even need all of that? If not, even a PIC10 will let you wake up from sleep on a pin transition.
In summary, many controllers will claim to be the best for low power, but your your exact application dictates which will be best suited.
Look into Dave Jone's uCurrent if you ever need to do really low current (nA) measurements.