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I have a lumsing one that I like. It is 10,000mAh.
Lumsing USB C External Battery Pack 10050 mAh, Quick Charge 3.0 Power Bank USB-C Battery Charger Type C Charging Block for nexus 6p iPhone Samsung and [link]
The thing I kind of like about the Lumsing is that it has 2 inputs. So if I'm carrying the correct cords, it can be charged quickly. The anker below, and an older version of that size Anker have been awesome for conferences and Disney but take a long time to charge. If you only need to charge one device, the one above is probably fine. If you want to be able to charge your phone and a tablet also, the bigger one is probably better. And the reason I have 2 options instead of 1.
And this Anker one:
Anker PowerCore+ 20100 USB-C, Ultra-High Capacity Premium Portable Charger, 20100mAh External Battery, 6A Output Type-C Port for Macbook, Nexus, Ninte [link]
I bought this one. Not very expensive, and it has performed perfectly.