Which is the most profitable gpu to mine?

I have some gtx 970 and i think they get the most kHs/Watt.

1120 kHs - 105Watts. Price: 150€ ~ 170/180$ (Second Hand)

And on the other hand you have:

GTX 1080Ti: 3100kH/s - 150Watts. Price: 780€ ~ 800/900$ (New)

GTX 1080: 2200kH/s - 120Watts. Price: 520€ ~ 610-650$ (New)

GTX 1070: 1600kH/s - 100Watts. Price: 420/460€ ~ 500/550$ (New)


Instead of buying one gtx 1070 you could get 3 GTX 970 and get the hashrate of the GTX 1080Ti for half the price...