The rx 570 in my opinion. Price is good and with oc and undervolting you'll get a sexy Hashrate with low Power cost...
YouTube has lots of Videos for this topic.
Sure, the RX 570 is cheap so it seems good to begin with. And we've actually begun by ordering six Sapphire RX 570 Pulse Mini's for the first test-rig. But how much can you undervolt it and what would the hash/watt be? That's the big and really important questions here.
We have the money but we don't have the power, that's why it's really important with hash/watt - and also the price for the GPU. As per my example in my first post - the GTX 1070 Katana use half of the power and has about half of the hash rate of the GTX 1080 Ti. So they're
almost the same when it comes to has/watt. However, since two GTX 1070 Katana's are more expensive than a single GTX 1080 Ti, it's not as good anyway.