If you want the absolute best video quality (4K/1080p High Bitrate) but the subtitles on the video are bad, use this method:
Use HandBrake (Free software). Load the MKV, go to the "Subtitles" tab, add your SRT file, and "Burn In." This creates a single file that always shows the subs (great for TV USB drives). If you want the absolute best video quality
If you want the absolute best video quality (4K/1080p High Bitrate) but the subtitles on the video are bad, use this method:
Use HandBrake (Free software). Load the MKV, go to the "Subtitles" tab, add your SRT file, and "Burn In." This creates a single file that always shows the subs (great for TV USB drives).