I personally have an I5-2500k, I'm looking at upgrading now as I'm beginning to notice the lack of performance (it's served me extremely well). For me the i5-7600k is looking a good option, a lot of articles have weighed heavily on their overclocking potential with some motherboard manufactures even incorporating a one click feature to overclock to 5Ghz which even air cooling supposedly can handle with ease (I've got a beefy air cooler so I'll deffo give this a bash!).