Choosing a USB battery pack / power bank depends on the device(s) you need to charge, how many times you need to be able to charge your devices while on the go, and finally, how much bulk and weight you can tolerate carrying around.

Air traveller should know that FAA allows passengers to bring spare batteries in carry-on bags, but not in checked luggage.

The best USB battery pack: TravelCard Charger (very small and thin, integrated Lightning or Micro-USB cable)

The best USB battery pack for everyday carry: Jackery Bolt (integrated Micro-USB and Lightning-connector cables)

The best USB battery pack with more power:  Anker PowerCore 20100 (ideal for long haul flights)

The best USB battery pack with quick Charge for compatible devices: Tronsmart Presto 10000 PBT10 (can charge your smartphone up to three times)