The best MK is the one that matches your playstyle. For example, for someone who likes to go in fast and hit hard with light combos, I would recommend picking the Mortal Kombat X version of MK11 because it has a shorter wait time between fatalities. Whereas those looking for more strategy might want to choose the Mortal Kombat 9 or 10 versions of their respective games as they have smaller windows during which you must perform an action such as blocking before being allowed back into battle