I can recommend the Anker brand. This is the one we got and it works great:
Anker® 2nd Gen Astro3 12000mAh (4A Output) Portable Charger Triple USB External Battery Pack with PowerIQ Technology
Keep in mind the mAh associated with the chargers, in relation to the mAh of your phone (or other devices) battery, determines how many charges you will get out of the device. So say you have a 12000mAh portable charger and your phones battery is 2100mAh. To figure out how many charges you can get out of it you'd want to deduct 20% of the original mAh and then divide by your phones battery; in this case (12000 * 0.8) / 2100 ~= 4.5 full charges. This gives you a rough idea of how many full charges you can get out of a portable charger; if your phone isn't full drained, obviously you can get more charges out of it.
Also, watch for how many amps each USB port on the device is. More and more smart phones are moving to use 2A charging ports. In the device I purchased, it has 3 ports and will charge at 4A between them; this means I can charge two 2A phones, or I could charge 1 2A device and 2 1A devices at the same time.
Finally, the larger device (mAh wise) you get the longer it takes to charge. I made sure the device I chose would be able to fully charge during the hours I typically sleep.