It turns out not all combinations work...
Of course I cannot test each and every combination, I simply haven't got the time to do so. I'm also a little short on certain devices but feel free to sponsor me 😁
This table shows different combinations that I am aware of, and (for many) have had the chance to test.
It looks like a good ol' HTPC (lots of effort required, and Microsoft may mess it up with yet another update) is the most versatile option.
If you want an of the shelf solution and plan something like a Plex homeserver then the combination of Nvidia Shield and Xbox One S returns the biggest bang for the buck. The Shield plays Plex 4k HDR and Atmos, the Xbox One S plays UHD dics and Netflix 4k.
When simply looking for a straightforward UHD player have a look at the Sony X700.
(Click on the image below to show a larger version of the table.)