This is the one I have and recommend. It'll charge your iPhone a few times and can even charge your iPad. It has two USB out ports and comes with multiple adapters albeit it does not contain a lightning connector.
It's hard to recommend anything unless you give more requirements. How much battery power do you want/need? How big in terms of physical size are you okay with? What's your price range?
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I'm looking into buying an external battery as well. The one I had charged my phone super slow. Is that how they normally work? Is there one that charges fast?
I've used my lithiumcard with my iPhone 5s, will continue to use it with my iPhone 6. Little battery that gives me an extra ~40-50% battery life boost, and small enough to carry around more often