Depends on what you mean by most profitable. In terms of price/hash and price/watt 1060s are pretty much top notch, but then you need to factor in additional hardware costs since you need more of them, which requires more motherboards, etc.
The 1080ti gets the highest actual hashrate, but you're paying 3-4 times the price of a 1060 even though you're only getting like a 30% higher hashrate. Some people say it doesn't matter, since once you earn back your original investment it's all profit and you should just go for the highest hashrate even if it's not the best price to performance. I personally disagree, I'd rather have a rig of 1070s or 1060s I can pay off in a few months that earns a little bit less over time. I can just pay that off and upgrade my hardware faster.