tldr; get a light 10.000mAh powerbank with 2A charging capability, included micro USB cable and an additional USB output.

When travelling, you need to bring a powerbank. You’ll probably use it before you even arrive since constantly being on your phone while waiting in an airport drains the battery really fast.

There are thousands of power banks out there ranging from 1 to ~8 charges for your phone with different features like fast charging (output and/or input), an LED-light or solar-cells – and a thousand tests that mostly ignore some of the following points. Here’s my suggestion on what to get for travelling:

• ignore the solar cells. They charge extremely slow and when you’re more than 3 nights away from any charging port, you should be enjoying nature instead of netflix anyway.
• capacity: 8.000-12.000 mAh. Smaller ones are only good for 1 charge; bigger ones get bulky and you’ll almost never need them
• included native cable for your phone (micro USB / USB C / lightning) – saves space and is really convenient; though it won’t last forever.
• at least 1 additional USB port (for a 2nd device or when your included cable’s plug gets worn after a few years)
• >2A output (if you have a USB-PD or quickcharge phone, get one with the same capability)
• >2A charging of the powerbank itself (if you have a USB-PD or Quickcharge charger, get a powerbank that supports this as well, e.g. with USB-C. Lighting inputs aren’t useful since you could still use your apple charger with a micro usb or usb-c cable)
• plastic case, preferably with a soft-pouch/bag to avoid scratches on other devices and safe weight
• make sure to read the 1 star recommendations to look for manufacturing problems or possibly burned(dangerous) powerbanks

Remeber that if your phone has a 3000mAh Battery, a 10.000mAh charger is most likely only able to recharge it ~2.5 times. That’s not because anyone is bad at math but because the energy gets converted from 3.7 to 5V and back – and you’ll probably keep your phone turned on while charging so it’ll burn through some battery already. Depending on how good your powerbank is, 0,85*0,85 up to 0,95*0,95 (72-90%) of your powerbanks capacity will end up in your phone.
In terms of technologies for the batteries, there are basically 2: round cells and puch-cells. Round-cells are used in e.g. many anker powerbanks and industry wide in a format called 18650 (18mm diameter, 65mm length). There are some manufacturers who use premium cells which are really sturdy and safe. The downside is the weight and wasted space between the round cells, as well as the thickness of at least 20mm (18+cover). Pouch-cells are flat (like the one inside your phone), lighter and more compact but you should make sure not to bend or try to break your powerbank.
For a powerbank that is ready for your next phone, I’d suggest USB-C input and output with USB-PD for charging the powerbank and the acessories you plug in