Completely subjective. Totally depends on how a team plays, and what tactics are used. Every position can be the most important at some point so there is no 'right' answer. However, if you change the question to 'in which position is it most important to have a good player?' then that is a little different, and you could probably say that 9 and 10 are the most critical because they direct, dictate and decide the play. But again it is subjective to each team because having one good player doesn't really add anything. Therefore the most important position is: Every position.