Go to the Opus One tour, it is really good. While the wine is very expensive, and the tasting is expensive as well, the tour is very nicely done.
While I have not been there yet, I am planning on doing a tour/tasting at Frog's Leap in the next couple of months. This one was highly recommended by a friend.
Also, try Reverie Winery on Diamond Mountain, it has been a few year since I was on their tour, but it was good, but not as good as Opus.