x264 (avc) & aac

1. ffmpeg -i "x" -c:v libx264 -crf 18 -c:a aac "x.mkv"

2. (delete original video file & assorted files like subs posters samples etc)

x265 (hevc) & aac

1. ffmpeg -i "x" -c:v libx265 -vtag hvc1 -crf 18 -c:a aac "x.mkv"

2. (delete original video file & assorted files like subs posters samples etc)

x265 (hevc) HW acceleration & aac & subs - bash loop

1. for f in *.mkv; do ffmpeg -hwaccel cuda -hwaccel_output_format cuda -i "$f" -c:v hevc_nvenc -vtag hvc1 -crf 23 -c:a copy -c:s copy -map 0 "${f%.*} HEVC.mkv"; done

2. (delete original video file & assorted files)