Best: Nvidia Shield Pro or Apple TV 4K.
Shield supports more video and audio codecs but depending on how you're using it that may not matter.
Shield Pro has a much better remote than the Apple TV which has a terrible remote.
Shield runs Android TV which may be a pro or a con depending on how you look at it, Apple TV obviously runs tvOS which has its quirks but is responsive and quite nice to use and just seems a lot more stable than Android.
What he said. Nvidia Shield Pro. I ran a NUC before and those who advocate for it obviously haven't used something like the Shield TV. It wasn't possible to get a NUC to support Dolby Vision and all the latest features like native refresh rates, etc. 4k content is a problem with NUC unless you pay like R15k+ or have a full fledged noisy PC in your living room. Integration between the NUC and TV isn't at the same level as the Shield and TV (which is perfectly seamless).
PC/NUC for multimedia is dead, you'll always get a lesser product because all the development by Netflix, media player software, etc. is going into Android TV and Apple TV. They just don't have the same level of support for PCs and since most consumers don't want a computer in their living room or want to pay 3x the price, it will never catch up again.
Nvidia Shield Pro does get updates contrary to some of the complaints above about TV boxes not getting updates.
I chose the Nvidia Shield Pro myself after looking for a NUC to replace my older model. To get one that could play 4k would cost R15k and then it couldn't even do everything the shield could. Then I looked at Apple TV but it was lagging behind the Nvidia Shield Pro. So choice is simple.