Just got back from Napa a couple of weeks ago. Went to Trespass, Larkmead, Turnbull and Chandon. To be honest I am a little tired of going to the bigger places where you have to elbow your way to the tasting counter only to have the server treat you like you're not worthy to taste their wines. Rarely have I had a good experience at a large winery.
I highly recommend Trespass. If you can, make an appointment with Don Gallagher, he is a gracious host and they make some really nice wines. Also, Larkmead was pretty nice too, they are bigger, but because they're appointment only, you don't have to deal with the elbow-shoving from the exclusionary types. Also, they're very nice there too. One more, if you can, and you have time, take a drive up to Neal, beautiful estate and caves, and once again, they're really nice (appointment only).