If you want to overclock the i7-6700k you will need a good air cooler / AIO / custom loop, I would say Air Cooler is the best bet. Anyways Cryorig makes great air coolers from cheap to expensive. I have the Cryorig r1 universal and have my 6700k at 4.8ghz at 1.3v so I got pretty lucky but still that's a nice performance boost just from my cooler. So not overclocking it I don't recommend.
If you absolutely don't want to overclock then a cheap Heat sink will do very good, Like the 212 evo. Can't go wrong with a 212 evo.
Higher end Heat sinks I'd suggest Cryorig , Noctua-D15 is a good one to.
also make sure you have a motherboard capable of overclocking it, Any z170 motherboard / z270 motherboard.