Boston MA has a VERY active social dance scene (mostly EC and WC Swing and more traditional ballroom, and some Hustle as well)

I'm biased because I used to live there, but I would suggest the San Francisco Bay area. The population is really concentrated, so there are lots of dance opportunites per square mile as compared to, for example, Vermont.

I always went to The Metronome Ballroom in San Francisco, but there are a couple of other large Ballrooms in the area. The Allegro Ballroom in Emeryville is at the opposite end of the Bay Bridge. The is also a Ballroom in the San Jose area, but I never went there and can't remember the name.

As far as Swing/Lindy Hop, Thursday Nights has the 9:20 Special, Saturday Nights has The Doghouse, and Sunday Mornings has Lindy in the Park (Golden Gate Park). For more information about the local Lindy dance scene in the bay, check out website for the Northern California Lindy Society at: www.NCLS.com.
Kevin

best cities for dancing based on what I've seen and heard are...

San Francisco, CA
Los Angeles, CA
Vancouver, BC
Kansas City, Kansas
Miami, FL
New York, NY

and like all of Europe from what I understand is like one giant hot spot of dance.

The question everyone should ask themselves before starting each day: Quo Vatamus