I think Artix-7 would be the best one/most cost efficient to mine BTC on, Virtex is a bit of overkill, and a lot more expensive.
Zynq would probably be amazing as well though (I think)
It seems to be a lot harder to get bitcoin miners to place and route successfully on Artix-7 than on Kintex-7.
Do you know, is that just because the Artix devices are smaller, or have Xilinx 'cost-optimised' the routing resources in Artix?