There is no doubt that the best gaming CPU is the i7-8700k, with the i5-8600k being a reasonably close 2nd place.
However at certain resolutions it is possible to hide this by running at settings which make the game performance limited by the GPU (aka GPU bound). For example, running at 4k with ultra settings in most games will demand enough of the video card that it won't be able to quite keep up. If you plan on buying a new CPU everytime you upgrade your video card, then it does not really matter what CPU you buy now if 4k/ultra is your goal. If on the other hand you plan on keeping the CPU for 1-2 GPU upgrades than at some point you will not be GPU bound and the CPU will matter again.
The other way to look at it is. What else do you want to do with your PC? Is the 8700k going to be the best overall choice for every activity? No. It won't suck at any of them, but for some a x299 platform or ryzen/threadripper would be a better fit. In some cases it comes down to money. The Ryzen line up has cheaper motherboards available and some of the CPUs are a very good value as well.