Best position is determined by the salary formula - nothing more, nothing less. Joesph Ka did a nice job of trying to nail down an estimate of it:
(46657.74)Basically, there is a formula for each position. Whatever formula gives the highest value for your player is the one that is used for salary purposes. It also detemines the best position for your player, but has no other impact on the game.
Run of the Mill Canadian Manager