If you buy a USB battery pack these days, it should probably have USB-C Power Delivery. The latest charging spec allows you to charge your phone faster, yes, but can also keep your tablet, Nintendo Switch, and even laptop charged up on the go.
Anker's palm-sized PowerCore 10000 PD Redux ($34, from the usual $46) features an 18 watt USB-C PD port that works both ways; it can charge your devices faster, but also lets you recharge the battery itself more quickly. 18 watts is the lowest wattage you'll see on a Power Delivery port, but it's still powerful enough to charge an iPhone from 0% to 50% in 30 minutes (with a compatible cable), and can even keep larger devices like laptops running for longer, even if it won't be able to quite keep up with your battery drain if you're actively using the computer.
Anker PowerCore 10000 PD Redux | $34 | Normally $46