Depends on your needs. If you just stream via Jellyfin/Plex I'd recommend Kodi+Linux on a raspberry pi or something. There are premade distributions like OSMC for this niche. If you need Netflix/Disney+ etc you're basically fucked on the FOSS front, the only option I know is either LineageOS's TV version though that isn't as well supported as the mobile version and there are some DRM shenanigans or just using Linux with a keyboard+mouse combo and a browser, though you will be limited by resolution. On the non FOSS front I have heard good things about the NVIDIA shield/apple tv though that is expensive, or using Windows with the native streaming apps and a frontend (eg Steam Big Picture).