Of these four, which is the best county to live in: San Mateo, Santa Clara, Marin, or Sonoma?

San Mateo county is far the best. I use to live in the area. There are a lot of things to like about the area aside form the landscape. This is where the airport is located, traffic is not so bad as I remember it. Housing is still good and lots of parks and good schools. Lots of good restaurants and part of the silicon valley can be found here. Public transportation is reliable, there's the BART, Santram and Caltrain.