I have 2 portable chargers:

New Trent iCarrier - Got on sale 2 years ago on Amazon. 12000mAh and lasts FOREVER. Apparentely has 1000 charge cycles, but I haven't even used it enough to test it. It's kind of bulky, not huge or anything, but hard to plug in and carry in your pocket without noticing the extra bulk and weight.

Mophie Power Reserve - Got it on sale on Prime Day last week. 1,350mAh and for me, provides about 60% extra battery life for my iPhone 6. It charged my phone from 40% to 65% WHILE running Pokémon GO without battery saver mode. It's small. Matchbox size, and the integrated cable is really nice. This is what I carry to work with me.