I don't have one yet but I picked this one out for myself:
Panasonic BQ-CC87ABBA
I have the Xtar PB2 and PB2S. I have a lot of 18650s and don't like cracking open devices to replace the lithium polymer batteries inside. Sure, LiPo outperforms cylindrical cells somewhat but as mentioned earlier the ability to quickly swap out the cells is a big advantage.
I also recently purchased a Panasonic BQ-CC87 charger which also functions as a powerbank.
Both the Xtar PB2S and the BQ-CC87 have an "always-on" mode which most powerbanks lack.
I actually keep all my lithium batteries at storage voltage except for two 18650s that I rotate through my headlamp. If the power goes out and my generator doesn't work for some reason, I have dozens of Eneloops that I can use to charge other devices. The Eneloops can be stored at 100% stage of charge unlike my lithium ion batteries.