There was a nice video app to watch .MKV with. Unfortunately it has been canceled by ungratefull assholes :/
The majority of media format is the .MKV and when an app doesn't support this format, you have to re-encode it to .MP4 which makes the output very big and the quality is never the same. So the best way is to re-encode .MKV to .MP4 is using MkvToMp4. What that app does is simply extracting the container, which is basically .MP4 without one single bitrate lost and resolution and quality is 100% the same, so 1:1.
Otherwise handbrake it is but it's still problematic as there is a chance you'll get errors like I have/had. I ended up watching them on my phone to save myself from headache and Sony with their cancerous memcards with limited space that are extremely expensive.