It is ratio of attributes not absolutes. You can have a HB listed as Balanced but have higher STR and BTK than some Power Backs. If the guy was slower he would be listed as a Power Back but his speed makes him Balanced. You can actually find bad backs listed as Power Backs and only raise there SPD rating and at a certain point (if it started low enough) they will become a Speed back.
Most important to a position? To his overall? If so just look for the attribute with the highest number, if you mean how he preforms on field... that would vary depending on how you use the player.