At my testing experience: 1080ti, 1070, 1060 and RX 480/580 are best gpus to mine ZCASH. I think the best one is 1080ti.
The best was palit gtx1070 with samsung memory doing 490sol@125wat- but it is impossible to find now and the price diff between gtx1070 and gtx1080 is very small.
Now I think it is gtx1080FE with 510sol@120watt after prices drop for this models.
It is all after fine-tuning
gtx1080ti is also good but expensive
Really cus my G1 does 500+ easy. Im pretty sure if you get your hands on a good 1070 it could come close to a 1080 maybe within 5%.
Mind you electricity for me is free though but I dont want to invest a lot (wont be buying the Vega cards anytime soon).
I'd stick with 1080Ti or 1070 and forget about the 1080.