I assume you're asking this question because you have expensive power?

Looks like most people like the 1080 Ti, and it is a strong hasher, but it depends.... on:

1. What algorithm are you going to mine? I only mine equihash, so that's all I can comment on right now. If you want to mine cryptonight etc, then disregard
On Equihash, a 1080 Ti can get 700-800 Sol/s, with the most efficient settings getting around 725 sol/s and using 250 watts.
A 1070 Ti can get 525-650 Sol/s, with the most efficient settings getting around 570 sol/s using 170 watts.

At those speeds, it comes down to the power cost usage over a month being only a few kW hours difference.

2. Purchase cost of the GPU - Some cards that are more efficient are so much more expensive to buy that you'd have to mine for 2 years in order to be at the same cost point so it just might not be worth looking for efficiency

3. Electricity cost - The more your electricity cost is, the quicker you can pay off paying more up front for a card in order to save power in the long run

Source: http://miningcharts.com/gpu-mining-comparison-chart/best-price-and-power-usage/
This comparison chart calculates, and visually shows you how much power you're saving, and how long it takes to make up the difference.

What I THINK makes the most difference is that this chart puts it into terms that make sense - it calculates power use and up-front cost per 100 hashes/sec so you can actually compare apples to apples!