AmazonBasics - the 2400mah high capacity ones. You can get them on subscribe and save for 10% off (if you buy a total quantity of 5 items of anything for that month's subscription).
I have a stupid number of them, more than 100; they work just as well or better as Eneloops, my previous brand of battery. I tried the Amazon ones on Prime Day or some such, when they had a big sale on all AmazonBasics stuff, and found that the flashes with the Hi Capacity AmazonBasics lasted about the same number of flash cycles as Eneloop Pros at 2550 mah. But, they cost less than half the price.
The power retention on them (when you just have them sit on a shelf) is good too.
I use them with 600EX-RT's, both Canon and Yongnuo, and with E3-RT's (where they last a very long time).