Anything labeled "WEB-DL" below 10 Mbps or files under 4GB for a 2-hour film—those will crush the grain into blocky artifacts.
You cannot download an MKV, but you can buy 300 in 4K Dolby Vision. Quality is good (15-25 Mbps), but it is encrypted. You can strip DRM with tools like TunesKit (legal gray area depending on country) to get an MP4/MKV. index of mkv 300 high quality