Newish and ON the beaten path, but not blitzed with cab-sauv tourists yet: Vino Velo, a tasting room cum bike jersey museum on Hwy 29 at the south end of St. Helena. It's owned by the family that owns CLIF Family vineyards (and CLIF Bars).
It's in an old farmhouse, has a nice garden, cool bike stuff, and the wine is very good, better than most of what the big wineries pour for the general public.
Tons of good info on this site. I did some research not that long ago and still have lots of threads saved:
Tons of topics on this already and recently. Please let us know your budget range per bottle, and whether you are willing to schedule appointments. Please check these threads out and let us know if you have further questions: