The biggest market teams are the Yankees and Red Sox, so they may be the easiest to follow. If you are looking to stray from the norm, the Cardinals, Cubs, and defending champion Phillies are all good choices. I would go with the Red Sox, because it has all the perks of being a Yankees fan without the cliche feeling you get whenever you see someone wearing a Yankees' hat for no reason other than they are the Yankees. I do realize this happens with the Red Sox now, but everybody seems to have a fixation on the pink hats of Red Sox Nation rather than the fashion hats.
