My recommendations:
Smallest pocketable charger - choose a "lipstick" sized charger around 3000 mAh capacity. Expect about one full recharge from it. This is the style I carry with me to the parks, since it's so tiny I barely notice I have it.
I like the Anker PowerCore. The DIS boards are replacing my URL with one to eBay at a higher price, but search
Amazon for ASIN
B005X1Y7I2- $9.99
Largest charger you can travel by air with - choose something at or under 10000 mAh capacity, as airline rules are beginning to block larger charging packs. Expect about three full recharges from this.
I like this Kmashi charger, because it's very cheap for the capacity, it's been reliable for me, and it has two charging ports. I bought four of them to power the centerpieces at my wedding because of the price, and ended up liking them better than many more expensive ones I owned.
http://www.amazon.com/KMASHI-10000mAh-External-Portable-Powerful/dp/B00JM59JPG - $13.99
Charger for the whole family - these get pretty hefty, but if you're already hauling around a stroller or large backpack, they've got the juice to recharge everyone's phone over the course of the day! The sweet spot in pricing seems to be around 20000 mAh (about 6-7 full charges)
This Anker is a good choice, but Amazon abounds with options:
http://www.amazon.com/Compact-20000mAh-Portable-Anker-PowerCore/dp/B00X5RV14Y
Fun trivia - the battery cells used in these battery packs are 18650 Lithium Ion cells - the exact same batteries used in Tesla electric cars. They're really powerful, reliable and convenient!
As for brands - you can't go wrong with Anker and they're the gold standard for cell phone accessories. But Jackery, Aukey and Ravpower are also brands I trust.