From my XP:
Ahash > Zpool > Hash Refinery

It felt like I was mining for free at Hash Refinery, sometimes hours would go by with zero updates, just lost mining, free mining, and this was with 75 Pascal GPUs (mostly this 1080 Ti) pointed at the same Hash Refinery algo. Zpool seemed to have consistently nerfed (under-performing) earnings, but was more reliable than Hash Refinery. Only recently moved to Ahash (almost 2-days), and it's been the best earnings out of the 3, make that 4 when including the time spent at Nicehash. I'm averaging > $9/GPU/day, perhaps a bit more than that since approx 10% of the time is spent at MPH (using Awesome Miner for profit switching). I've only been mining a few months, so I'm also trying to figure out what's best. I've come to the conclusion that to scale larger and maximize earnings, we must constantly research and adapt.