Metronome Ballroom -- www.metronomeballroom.com
Pick Studio -- www.pickdance.com
Allegro Ballroom -- www.allegroballroom.com
Imperial Dance Club (although this is a bit more focussed on competition dancers) -- www.imperialdance.com
Of these, my favorite is the Metronome. It's in Portreo Hill and has an extensive social dance program and plenty of free parking. My second favorite is the Allegro, as they also have lots of free parking and a large selection of group classes to choose from.
If you decided to go with a place like Fred Astaire or Arthur Murray, make sure you fully understand their "contract" costs, what you have to pay up front, and what you have to pay if you decide to break the contract. The Independent studios I list above don't have their students sign contracts, the group classes and even most private lessons are taught on a pay-as-you-go basis, so you rarely have to ever put up more than what four weeks of group classes woud cost in advance (see web sites for group class costs and schedules).