Obviously, one person's "best" might not be another's. Depends on how you want to make or consume the music. If you're looking for a place where you can go to see and hear performances in concerts, clubs and festivals, you might not care about finding a place with an active song circle or regular jam sessions. If you're a singer, a great old-timey scene might do nothing for you. Or if your taste in songs runs toward singer-songwriters, then shanteys and other participatory stuff will not make you happy.

Another element in the mix is radio. Is there a local radio show or station that plays local artists and announces local events? Not those canned NPR shows like "Thistle and Shamrock" and "American Routes", but a show that's produced and presented by people who are part of the local scene. Lots of people won't pay to hear an act they've never heard of, but they'll go after they've heard somebody on the radio.

That said, I'd agree with the list of cities already in this thread--Boston, New York, Philadelphia, Washington, Chicago, maybe Denver, and much of the west coast are all good places to find active folk music communities.