For a 2 cells powerbank, I would recommend the XTAR PB2. Best one yet.
For a much bigger powerbank, I would recommend:
I’ve tested it, and it is the only one which cuts off charging at always 4.19-4.21V. Never goes higher, and never goes lower. It can also charge at up to 1.95A(confirmed by my USB tester), and can output up to 4A before shutting down. Out of many other DIY chinese powerbanks that I bought and tested, it is the only one I actually trust my life with it, and sold it to many of my friends with tested cells.
It also has a reasonable efficiency of 88% at 5V 1A, and 86% at 5V2A, as per advertised specs and per port. You can’t easily remove cells though, but if something dies, you can actually remove it without any tools, unlike sealed powerbanks
For the ultimate DIY powerbank, I would recommend this monster, if you can easily buy it though, and can stomach the price: