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