I'm also throwing in a vote for the R7 3700X, however you may want to also consider the R7 3800X. I have now built a few 3700X systems and a couple 3800X systems and there is actually a valid reason to consider the 3800X.
First of all PBO with the Ryzen 3000 series are just about what I would consider broken. It no longer gives you the maximum system performance like was the case with Ryzen+ (2000 series). Now you have to manually overclock via multiplier overclocking, but the good news is manually overclocking with Ryzen 3000 is extremely simple. With that said most 3700X processors are going to top off at 4.175 - 4.3Ghz all core (depending on your luck with the silicon lottery). Most 3800X processors are going to top off at 4.3 - 4.4Ghz all core and I just finished one that I wish I was keeping that hit 4.45Ghz @ 1.36V. The 3800X is a better binned version of the 3700X and can typically be overclocked higher.
My personal rig is running a Ryzen 7 3800X @ 4.4Ghz and I can attest through a lot of testing that I get better performance in productivity (rendering, editing, ect) and the same gaming performance (within the margin of error) of an i9 9900K @ 5Ghz. I know that seems to be a "bold" thing to say, but it is true. AMD now has better IPC than Intel in the current generation and the only way Intel can hang onto that "gaming king" title is by hitting 5Ghz and beyond. But most of the reviews your going to see of the Ryzen 9 3900X and Ryzen 7 3700X are based on the processors running ~ 4.2 - 4.3Ghz. In those reviews they give the i9 9900K ~ 6% better gaming performance and that is only if you are using the top of the line RTX 2080Ti. When you pit the R7 3800X @ 4.4Ghz (with properly configured RAM - 3600Mhz CL14) against the i9 9900K you see that 6% advantage to Intel virtually disappear. Its also important to note that I'm hitting 4.4Ghz all core with my 3800X using a NH-U14S cooler and under load my temps hit 73C as a high (while rendering, gaming I have yet to see a temp exceede 62C). To hit 5Ghz or beyond with an i9 9900K you are going to need a quality AIO or custom loop cooling to keep that power hungry processor cool, which will add more cost to your rig.
Also important to keep in mind with gaming that the only time your going to see the i9 9900K outperform even the R7 3700X is when you are running the top of the line RTX 2080Ti. People don't seem to realize that if they are running a mid range graphics card there is going to be no difference in gaming between the 3700X and 9900K as the GPU will be the bottleneck. Its only when you are using the top tier RTX 2080Ti that you see the 6% gaming difference between the 9900K and 3700X, if your using any other GPU then the two CPUs are equal in gaming performance.
Really I would highly recommend either the R7 3700X or the R7 3800X over the more expensive, more power hungry, hotter running i9 9900K.