Why does it matter? I think there are LOT of people who would say Boston, and the fact that there are discussions like this going on among Boston fans just supports their case.
Is it arrogance when you really are the best fans? Or when your team is obviously great?
For years I've said (HUGE generalization) that Celtics fans know more about basketball than the fans of any other NBA team. I think Barkley said it once too.
Anyway, as a Dallas Cowboy fan once forced to live in the DC area, I say that 1980s Redskins fans win for pro sports arrogance.