I have got powerbanks of 2000mah-27000mah.
I have business where average calls are 100/day and lot of PDF files, plus im always on the move. So phone battery lasts maximum 7am-3pm.
After years of experiment, i realised best is to carry two powerbanks of 10000mah each.
MI 10000mAh Lithium Ion, Lithium Polymer Power Bank Pocket Pro with 22.5 Watt Fast Charging, Dual Input Ports(Micro-USB and Type C), Triple Output Ports, (Black)
https://amzn.eu/d/36dGp5N
One i have this. Other one is also Mi, slim version. This way, 1 powerbank charges while other im carrying. And vice versa. I hardly put my phone on wall socket unless im at home..
To conclude, In my opinion, powerbank being pocketable, light weight and fast charge, with good charge-hold is really important.
Plus it should be able to charge low power devices like Bluetooth TWS, Watch etc.