Lithium Polymer/LiPo set to power Arduinos
If you can get away with it, I recommend not using a boost converter and instead connecting the battery directly to the supply voltage input (the "5V" pin).

The AVR chips used in most Arduinos (but not the Due or Zero [and the Yún and Tre are a totally different kettle of fish entirely]) can operate with anywhere between 2.7 and 5.5 volts, with the ATmega328P found in the Uno able to run with as little as 1.8V. This is a much larger range than the 3.0-4.2V available from rechargeable Li-ion/Li-poly/LiFePO4 cells.

Naturally you will need to make sure that any external components can run with the reduced voltage as well, but quite a few are able to work with anything from 2.0 to 5.5 volts.