Depends what you're carrying in.
I own the Powercore+ Mini, the Powercore 5000, Powercore 5000 Slim, Powercore II 6700, Powercore 10000 and the Powercore 26800, and the Powercore Fusion.
For in a trouser pocket I find I like the Powercore 5000 Slim, it is flat, flush against the back of the phone is a reasonable pocket fit.
For a fanny pack / bug bag / belt bag type then the Mini or the 6700 as they are densest power/weight/volume and enough power to cover a long day.
For backpack I always use a 10000 as it is the best overall power/weight/volume and can keep phone and a tablet going.