in Napa, I'm a big fan of Oakville and the Silverado Trail.
Cakebread is great as a first stop. They require reservations for an indepth tasting. They even provide recipes for foods that go well with the wines they pour.
Opus is worth a visit once but at $25 for a small taste, I would rather just buy a bottle at a store and drink it at home.
Silver Oak has always been a favorite. Groth too, but you need a reservation. Rudd is very good but expensive. Caymus is good, as well as ZD. Rombauer is further on up, but very good too.
I thought Mondavi was good for a tour but is way too crowded.
All of these are larger wineries with good reputations and you cant go wrong. Plus, being off the main road, you avoid traffic. There are tons of smaller vineyards to check out as well. Just pull in if one catches your eye and see if they'll pour for you.
Here is a good map:
http://www.napavalley.com/visitorsinfo/Napa_Valley_Winery_Map.pdfFor Sonoma, a good first time visit would be the wineries just north of Sonoma on 12. Chateau St. Jean, Ledson, Landmark, St Francis, Arrowood...they are all good.
For some smaller wineries, head north on 101 to Alexander Valley. i like Jordan, White Oak, and Lancaster (Roth wines are made their too).
Here is that map:
http://www.sonoma.com/visitorsinfo/Sonoma_County_Winery_Map.pdf