1080 is no good

On equihash
1060 270/sec
1070 430/sec
1080 ti  635/sec

On amazon current prices
1060 $300
1070. $500
1080 ti $750

Rate / price
1060 .90
1070 .86
1080ti  .85

Assuming the above are correct the 1080ti is the best bang for the buck.

You also have to consider what scale you are going to mine on. Motherboards, power supplies, risers, etc are not cheap. One rig with six 1080ti would certainly be more cost effective than building two rigs with six 1060.