There are different contexts where different products fit.
The most common daily situation I have is I have some kind of bag to carry tablet, clothes, food,water, etc so I'm not that bothered about power's weight/size but just need a lot of flexibility. For those typical days I carry the Powercore 10000 as it has enough power to handle the one worst day I could have of using tablet+phone all day heavily with never time to plug into mains. I then also carry the Powerport 2 as it can full power charge 2 things at once.
Less common situation is I'm moving with no bag, just my pockets, and for these I may carry the Powercore Slim (5000mah) as it fits well into pockets, this can make the worst possible day my phone handle everything I may do that day.
I love my Soundcore IE20 headphones, I have them around my neck and plug into ears if someone phones as it keeps my hands free.
Rarest situation is a long time off-grid, and for that I can either carry a few batteries (say my 5000+10000mah batteries) but I won the 9am PDT daily offer of the Powercore 26800 which means I could carry just that. Not so sure that is wise as if I were days off-grid I'd not want 1 battery which can fail but multiple batteries and much less likely all could fail.
I maintain a spreadsheet of all of Anker's products with their relative size, weight, capacity, power input/output. From that the Powercore 10000 stands out as the obvious best product if you wanted to buy just 1.