I've lived in the NYC area my whole life, so it's hard for me to pick another city since the combination of volume, passion, knowledge is just so rich.
Within the city limits, there are unique epicenters of fandom -- imagine the fandom palette of someone from the Bronx vs. Queens. There's the four key pro sports, college fandom, even flavors from across the world.
Of course, Boston, Philadelphia and Chicago have to be up there too. If I had to rank those cities, I'd go Chicago, Philadelphia then Boston. Out of all of those cities, Boston feels the most "bandwagon-y" to me. Maybe because it's the smallest city out of the four mentioned above, but the proportion of true fanatics, to me, feels less proportional than the other three. More objectively speaking, college sports is less impactful in Boston than the other cities. Elite hockey, but basketball and football in Boston is a dud.
More importantly, if you think your city is the best, then roll with that. Plenty of smaller cities with insanely passionate fanbases (I'm looking at you Fargo, ND and Cow Dick, TX).