Best hardware:
GTX 1060 - 270-320 h/s
GTX 1070 - 400-480 h/s
GTX 1080 - 500 - 560 h/s
GTX 1080Ti - 700 + h/s
Find which one is best price / hash. Those are rough estimates, but you can get more info around.
The 3GB 1060 offers the best price/hashrate, but you lose out on rig density in comparison to more powerful cards, especially the 1080ti. Always an important balance to consider.