With Intel being "better for gaming" etc, I think for people who upgrade every year or so and don't really care about cash the 9700K is a good choice for gaming, but if you're going to keep the CPU/board for 3+ years then the extra cores of the 3700x or 3900x are almost certainly going to work out better.
The 9900K seems harder to justify, as games aren't going to need more than 8 high clocked cores within 2 years, the 9700K at the same clock is basically the same in gaming, and the 3900x beats it in every other way.