I use this one it is a bit weighty, but can charge my phone from dead to full like 7 times.
Grab one of these too (unless you have an iDevice). The relatively short length (1') and low AWG/resistance means your phone will able to draw more current from the battery pack.
Many battery packs, especially Anker's packs, will output up to at least 2 amps per USB port. If you're using the right charging cable, like the ones I linked to, you'll be able to max out the current draw and charge your phone as fast as the laws of physics will allow you to.