If you don't need Windows Media Center, any version of Windows past Vista should do just fine. As far as Microsoft is concerned, they have interest in supporting only Windows 10. As far as building with new equipment, Windows 8.1 & 10 can install on new hardware with a minimum of fuss, but Microsoft is throwing up monkey wrenches when it comes to support for new equipment on anything other than 10. As far as OS overhead and a minimum of downtime from pointlessly major updates, 8.1 runs faster on less equipment, despite Windows 10 being able to run in a smaller memory footprint.
If you need UltraHD Blu-ray playback support, currently only the Kaby Lake platform is supported, and you guessed it, that means Windows 10.