Great thread. So to summarize based on the results of this thread and what I've found, this seems to be the possible options for replacement batteries:
* iFixit: High variance in batch quality. There are also reports of weird things like the manufacturer changing after a few cycles from iFixit back to SMP which makes me question the quality of their BMC. They claim to do extensive battery testing [1] but it doesn't seem like that's doing anything to prevent bad batteries.
* NewerTech (OWC): Possibly less variance than iFixit in terms of quality, but still no guarantee of a good battery and likely not as long-lasting as apple OEM.
* 2Power (DuracellDirect, UK): Haven't seen too much about this online, and there are both good and bad sentiments for the few comments that do exist.
* REWA: At least one good comment about it on this thread, and far from being a random chinese no-name company REWA is decently well known in the apple repair scene. There is a video of the QC process for their iPhone batteries [2]. I suppose since all these come out of China anyway you may as well buy from them and cut out the middle-man.
* LMP: This is a company I have not seen mentioned at all, and it seems their products are mainly available in Swiss countries? There is one teardown [3] which has good things to say about the quality of their BMC. I found one reddit post about someone who used this battery, and when I reached out he mentioned that the first replacement lost capacity quickly after ~3 months but the second replacement has held on so far (~6 month).
* Ebay: Best avoided for retina mbp batteries? I'm very skeptical that there are any genuine apple batteries on there considering that no gray-market versions are available they are only available as part of the top-case to aasp, and removing it from a donor laptop is not a clean process.
@startergo How is the REWA battery lasting so far? What does the manufacturer in CoconutBattery indicate?
What I'm curious is what brand of replacement batteries 3rd party repair shops use. They would be in the best position to get a feel for which brands are of good quality. I tried searching to see if Louis Rossman had said anything but in one of his video comments he mentioned he just "skypes a guy in China who runs around to do his bidding." If any of you have friendly repair technicians nearby, maybe you can try asking?
[1]
https://www.ifixit.com/News/34324/how-we-test-the-quality-of-batteries-we-sell-at-ifixit
[2]
[3]
https://logi.wiki/index.php/MacBook..._.28Late_2013.2C_Mid_2014.29_Model:_LMP_A1582