6700K (which is unlocked and can be OC'd), since you already have the Z170 (overclockable) mobo. Games of today are using more threads than before, so, despite the 4C/8T of the i7 previously intended for multi-tasking, it is now considered a basic CPU in this day and age (esp. with the release of 6C Intel Coffee Lake CPUs and 6C AMD Ryzens)