I have an Nvidia Shield, I don't use it because it doesn't work properly. I'm very disappointed by it. With that being said, what I need is a box that will play 4k movies, shows, videos, x265 10bit. I download my sources, and have an external USB which I put the file on, then I would like to just plug the USB(3.0) into the box and watch what is on the USB. Any recommendations? Thanks.
The Shield would be my biggest recommendation there. What kind of issues are you having with it?
The only other box I've used is the Jetstream 4k Android box...I picked it up at Walmart for about $70. I use it in the bedroom and my Nvidia Shield in the living room where the family tv is. To be honest, there is only a slight bit of difference as far as using it just for tv which is all I do anyway.
Without listing what the Shield is not doing for you its hard to make a recommendation. Because it should do the things you have listed.
The shield or mi box, contact Nvidia for help
If you only want to download and play x265, do not waste $150 on a Shield. Buy a S905X2 box and install CoreELEC. It will play whatever you need perfectly with kodi. Also no problems with IPTV interlacing. It will cost you $35. If you want Android TV I would get the Mi Box. People are obsessed with the Shield here, but if you really just want to play USB MKV's get a S905X2 box with Coreelec.
Last answer you want but get a new shield. nothing else will come close to playing all the file types you need.