[wrxtasy]

OK, so after going over the details of the recommendation on the first page, I think my demands are a little bit different.
I want LibreELEC OS (not OSMC OS - because they are very slow in version release), 4k HDR 60FPS with HEVC 10-bit playback support.
Dolby Atmos and DTS-X, and I will run clearart heavy skin so I need smooth experience, what box should I get?

Usually, NUC was the best option but because of bad HDR support, it is out of the table...

Difficult to say - likely the only option is a NVIDIA Shield due to it fast GPU, and then deal with Android updates and not a lot of Android Kodi support.
Auto colorspace switching and such settings are still Beta on that device and buried deep in Android developer options.
Users still appear to be having difficulties with getting this switching working reliably.

The AML S9xx devices (CoreELEC / OSMC) have easy either auto or minor set and forget colorspace switching settings, but lack outputting Correct - MaxCLL/FLL HDR metadata with their v3.14 Linux Kernel.
The S912 chipset is the fastest in that group for demanding Skins.

The new ODROID N2 running CoreELEC Kodi is looking promising with a pretty decent GPU for heavy skins and I believe outputs Correct - MaxCLL/FLL HDR metadata with it's newer v4.9 Linux Kernel, but it is not what I call stable yet for 4K HDR. It's also missing multichannel LPCM audio output.

The ultra smoothest Kodi Skin performance is going to be a NUC.
As I said difficult to choose. Nearly every device has either frustrations or features not quite working properly.

Forget about LibreELEC and HDR for Kodi Leia - comes back in about 8-12 months to see what progress has been made for v19 Kodi M and HDR.