Based on your specs, the 1080 would offer the biggest performance boost compared to any other upgrade that would be less than 300.
While a more modern i5 or i7 (such as the 6600k or 6700k) would offer a CPU/bottleneck opening boost of 15%-20%, the new RAM, CPU and Motherboard would cost you a minimum of $650, if you cheaped out on the parts and bought on sale.
A 1080 is ~20% faster than a 1070 anyway, so the extra 300 would give you an equivalent boost as if you bought a whole new base-array. That said, the early i-series stuff is getting a bit long in the tooth (though not long enough to merit running out right now to upgrade) so it's something to consider probably next year if you're not a year-to-year upgrade guy.
Don't buy a 1080 "Founder's Edition" (FE) if you can. There are alot of great options for aftermarket cards. My best recommendation is ordering the (NewEgg exclusive unfortunately)
MSI Gaming 8G (not the Gaming X). It's the same card/cooler as the Gaming X/Z, however it's pre-OC is a bit lower (but can handle the high clocks if you manually tune it). It's ugly as sin but the cooler is great. The
EVGA 1080 SC is a cheaper option that still handles good overclocks however.
[insert segueway here]
Speaking of z170 socket motherboards, the ASUS Z170-A I bought last year (literally about 13 months ago) died two nights ago, very suddenly, and would not boot (or even post, or give error messages). After testing/part swapping, I did finger the board, and promptly went out and bought the
MSI Gaming Pro Carbon which as it turns out, is a pretty snazzy and solid board. Good wafer weight, hidden soldering, and the BIOS features are great.
3 out of the last 4 ASUS motherboards have died on me over the last 6 or 7 years, so I'm pretty done with them. MSI is starting to become my favorite hardware brand...