Some iPhone users like to stick to official Apple products only. That way, everything's certain to play together nicely, and if it doesn't, those geniuses at the Apple Store are sure to help you fix it. Okay, that's going to be more expensive, but if your time is valuable, or you just want the minimum of stress, it can be a price worth paying.
If you agree, then your best iPhone power bank is the MagSafe Battery Pack, an official product from Apple. Note, though, that it's expensive, and the battery isn't the highest powered, at just 1,460 or 2,920mAH.