Something to understand with these batteries is that there is an inevitable loss of efficiency with the internal circuitry. You won't get 10,000 mAh out of a 10k battery...most testing that I've read has been somewhere around 65% efficiency, or about 6k mAh from a 10k battery. You will also loose some capacity as the cells age, further compounding the useable mAh/ounce ratio.
I've had a couple Zendure batteries since they were a kickstarter several years ago. They advertise having the most efficient circuitry on the market. I can't really attest to that because I've never tested them against anything else. I can say that in the last few months, I've run several tests on my ~4 year old, heavily used Zendure A3 and Gridder 1, and both have consistently exceeded 70% efficiency by a a couple percent.
I bought a Zendure A1 a couple weeks ago to take on my backpacking trips. It weighs 2.4 ounces and I've run a few tests under different conditions and have been getting a bit over 2,400 mAh out of it. That works out to right at 1,000 mAh/ounce. My A3 works out to pretty much the same, even after several years of use...which tells me the construction of it is of enough quality that it's really not losing efficiency as it ages.
My personal testing with that A1 coincides with my math, and I get at least a 60% charge from it on my GS8 Active (4,000 mAh battery). I've found this new phone to work fine for 3-5 days on a full charge if I leave it on airplane and battery save mode, even using it to take a good amount of pictures and leaving the Always-On display on. The 60% boost I get by plugging in the A1 will stretch me to about 5-7 days, which is really all I need it to do. The A2 at 6,700 mAh (I predict 4,700 mAh useable) would give me a full charge and then some, but at 4.8 ounces I don't really feel the need for it unless I start doing some heavy battery consumption stuff while I'm backpacking. It looks to me like Zendure is sizing all of the A series to hit that 1,000 useable mAh/ounce, so they're pretty easy to shop for if you know what your consumption will be.
I can also say that Zendure's claimed weights are accurate down to the tenth of an ounce with the three that I own. They also support charge-through, which I understand is not a feature that all batteries offer. It's nice to be able to use one outlet and leave things charging overnight, or leave several batteries plugged in to your truck while you leave, and come back to them all charged.
The only complaint I have with Zendure is that they haven't released a USB-C version of any of their batteries yet, so they would match my new GS8 active. I think that connector just came out a few months ago though, so I'm sure it will become more mainstream in the near future.
The only Anker products I have experience with are their replacement batteries for Samsung phones, and I've had no complaints. They worked just the same as the original Samsung batteries. Their construction doesn't look as durable as my Zendure batteries have proven to be, but that's just from looking at their advertising and pictures and is just heresay on my part. It's not like I strap any electronics to the outside of my pack either, so as long as they'll handle drops and occasional accidental moisture, it probably doesn't make a difference anyway.