.x265

Best compression for video formats as of now. Can be used with the mp4 or mkv container formats.

ffmpeg -i INPUT -an -x265-params crf=25 OUT.mp4

You can also mux it in Matroska (.mkv):

ffmpeg -i INPUT -an -x265-params crf=25 OUT.mkv

ffmpeg -i input -c:v libx265 -c:a copy output.mp4