With Skylake now available, there is absolutely no question in my mind that a new build should be Skylake.
a. Prices for cpu, z170 motherboard and ddr4 ram are almost precisely the same.
b. 6600K has an estimated 5-10% performance improvement per clock over haswell.
c. 14nm runs cooler, you get a decent overclock without the need for exotic cooling.
d. The Z170 chipset permits the use of much faster ssd devices on the horizon. Samsung 950 pro for example:
http://www.samsung.com/global/business/semiconductor/minisite/SSD/global/html/ssd950pro/overview.html
e. skylake can be upgraded in the future to kaby lake.
For gaming, the i5-6600K is just as effective as the more expensive i7-6700K.
They both will oc to the same 4.6+ level.
Few games will use more than 2-3 cores so the hyperthreads of a i7 are largely irrelevant.