There is another option when it *really* has to be low power - disconnect the divider in between readings. If you turn on the divider and any buffer amp for 60ms once a minute, you can reduce its drain on the battery by a factor of 1000. Also, for applications with soft power switches, you can virtually eliminate the extra power-down consumption of the battery monitor circuit, preventing excessive battery discharge in storage.
One way to do this is to use an OPAMP with chip select e.g.
MCP6043. Pull /CS high and it turns off, with its output hi-Z.
If you power it from the LiPO cell you are monitoring, and connect both the lower end of the input divider and the /CS pin to 0V via a logic level small signal N-channel MOSFET (e.g. BSS123), you can switch it off *AND* disconnect the divider under MCU control. Add a pulldown on the gate and its also going to be off when the MCU is powered down.
However if you are careless about the feedback its output could spike to its supply rail during turn-on, so you may need a series resistor between its output and the ADC, and possibly clamping and/or a capacitor to 0V at the ADC pin to provide low pass filtering and to keep the pin voltage from rising excessively during turn-on.
A safer way to disconnect the divider is to put a P-channel MOSFET at the top end, with a pullup on its gate and pull the gate down with a logic level N-channel MOSFET driven by the MCU. That lets you run a micropower buffer OPAMP (if required) from the MCU's supply rail as its input wont be above the rail when the divider is disconnected, and also its output cant drive outside the rails so ADC input protection probably wont be needed.