The Core i9-9900K still offers the highest level of performance for gaming builds. There are chips that come very close to its performance, and others that offer far better value, but for no non-sense peak gaming performance, the power hungry 9900K is as good as it gets.
Builders should note Intel is about to release a newer Core i9-10900K CPU that will be based on similar architecture but is supposedly bringing more refinements and a sizeable performance jump. Once it's out and we can review it, it's quite likely we'll be recommending that instead.
For pure gaming builds, you may also consider the older Core i7-8700K or 9700K which are much cheaper -- they come close, often testing within a few percentage points of their pricier cousin. The 8700K is also remarkably well behaved for a CPU running at over 4GHz by default. Things get even more complicated when you consider motherboard compatibility (the 9900K demands a high-end board) relative thermals, power consumption, and overclockability. Dollar-for-dollar, the 8700k is arguably the better choice for most.
Extreme PC builders should also take a look at the Ryzen 9 3950X. It’s a faster CPU for work and very fast for gaming though it costs quite a bit more at $750. Also worth giving a nod to the Ryzen 3700X. The chip will set you back about $310, while still offering solid gaming performance. Additionally, if you use your PC for any productivity undertakings, you’ll find that the 3700X surpasses the 8700K and closes the gap on the 9900K.
However, if all you care about is pure gaming chops and nothing else, the 9900K is the clear-cut option, and should set you up nicely for some CPU-intensive upcoming games like Cyberpunk 2077.