I often go to Simply Balanced (www.simplybalanced.com) on California St in SF for Pilates classes.
My favorite teacher is Susan Powers, she is incredibly knowledgeable and funny. In addition to Pilates she is also trained in The Franklin Method (www.franklin-methode.ch), which is a great approach to anatomy and very interesting to apply to Pilates.
I take private sessions with Susan and I try to attend the mat class, which is scheduled daily except sunday.. This is one of the best mat classes you can take, in San Francisco. Susan is very "hawk-eyed", she sees everything, lets you get away with some, and she knows her stuff.