If you look at the best town as a combination of success and how good the fans/purely how important sports are to the city then the top 3 has to be some order of Philly, Chicago, and Boston. New York and LA have both been very successful, but LA fans just aren't that great, and New York just has too many teams to have the same passionate fanbase as seen in places like Boston, Philly, or Chicago.
As a Bostonian born and bred, I'm obviously partial to Boston, but, at least for me, it's a toss up between Philly, Chicago, and Boston.