I had AMB power bank with capacity of 23000mah and it was amazing. You can charge upto 5 mobiles easily from 0 to 100
Although its size is big and has some extra weight but i haven't seen any other power bank with such a great performance(excluding samsung's original one).
I m regular user of powerbank so i have tried too many brands
Note: now using original samsung's powerbank which is the best...