What should i upgrade next?

[canadianpawn]

I have an gtx 1070 8gb
i5 6660k 3.9ghz
asus h170 pro gaming
16gb ddr4 2133mhz
h55 liquid cooler
240 SSD Kingston
2tb hardrive
850RM Gold PSU

 

[Lutfij]

Logically getting a Z170 board would be your best route. If you want the latest then go for the Z270 boards though you will miss out on Intel Optane support. The next thing to do is get ram that is at least DDR4-3200MHz. The K suffixed processor part and the Z series chipset will allow you to overclock the ram.

 

[canadianpawn]

Once Coffe lake will come out. Then i'll upgrade.

 

[geofelt]

If you have money to spend, consider a better/added monitor.

As to cpu, I think I would put any cpu upgrades off until Canon lake 10nm, supposedly in early 1018.
There will be new 300 series chipsets.