I think who has the most PIMs will only give an indication as to who a teams enforcer is (Avery is an exception), but I think just purely looking at the end of a season who has the most PIMs wont tell you who an enforcer is. Over the period of a season you would need to look at each individual game and each individual fight/roughing to see what event merited the enforcer going out to do do his job. Other team tking liberty's with your star guy? Dirty hit on one of your skilled players? etc...
I think an enforcers role is also to change the mood of a game, your team is 2-0 down, your at home, you need to get the crowd back into the game. You put your enforcer on, hopefully he kicks a** and then that momentum is taken forward to possibly spur your team on to score.
