Yeh well, obviously the backs get to score most of the points- the halfbacks and the centres are your generals in deciding exactly where the ball goes- but what exactly would they do without the ball?
IMO, the most important player on the field has to be the openside flanker, usually he will have the highest tackle count in any match, he will have to win turnover ball which in turn can release the counter-attacking backs, he will have to slow down the opposition`s ball when he can`t win the turnover, he will have to contest for every ball on the park. Without a good openside flanker, your team is dead and buried before you have even begun. And I say this as an inside centre!