I'll also throw a vote for Anker. Reasonably priced, variety of sizes (physically and power-wise).

I have this one and it has held up to almost 2 years of usage. Small size, decent power reserves:

https://www.amazon.com/gp/product/B00P7N0320/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1

I also have an iMuto. I hear them talked about a lot less than Anker, and I bought this one on a "Lightning Deal" on a whim, though I don't recall the exact price, but I'd buy another one at 30 bucks if I needed one. It's bigger than the anker and bigger/heavier than a phone, but it fits in my pocket still and is more than up to the task of keeping a phone going all day:

https://www.amazon.com/gp/product/B01191S0VY/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

EDIT: and the second charge port comes in really handy sometimes. I've been able to bail out other trainers at raids who need a charge but forgot their charger even when I'm charging, too. Also useful at community days if you don't mind being tethered to somebody else while walking around.