It depends on whether you want to go north or south. Carrie Coe and Pamela have a barn in Portola Valley, Seven Oaks. They've both been around for a long time and do a good job. They're some of the good ones. Terry Church up north doesn't really show, but she's good.
Also Laurie Moore! She is excellent, has horses you can ride. She comes to our barn once a month and wish it were once a week! http://lmsdressage.com/
Also, I second Seven Oaks. If I were not leasing a horse, I would still ride there. Lots of awesome horses, very cool instructors. They love love teaching, will lunge you if you wish. Lots of affordable packages for lessons.
Was just an article in Riding Magazine about a trainer north of SF, has a few nice sounding school horses including a 4th level school horse. Natalie Pitts - she does eventing and dressage. Hard to find instruction with school horses, so worth checking out.