If you had your choice of a California wine region to visit, would you choose Napa Valley, Sonoma County, Lake County or Mendocino County?
Well, fortunately I have been to all of the above and to the Central Valley and Central Coastal regions, too. My wife's sons live out there. Every time we visit them, we also take time to go to a wine region and do some tastings.
Each place you mentioned is a completely different experience. Over a series of trips, I would try to visit each. Napa and Southern Sonoma are pretty close to the same, with Sonoma a little less tense.
To Start:
I would say, start with Napa and Southern Sonoma. They are very touristy and fun. They are getting pretty snobby there, like France has been for a longer time. That is expected, I guess; making great wine can do that to you...
As I have written before, I enjoyed Northern Sonoma (Santa Rosa and up) very much. There are some big names there like Rodney Strong and Kendall Jackson, but there also are lots of smaller boutique wineries there that make some really great (but expensive) wines. You can't buy their wines in any store.
I loved Mendocino county. Lots of Pinot Noir and Zin. When we last went three months ago, it was not unusual to taste 5 or 6 year old Pinot Noir, which typically you don't see that old. It has to be great Pinot to age well that long. I really like Pinot Noir, so I was in heaven.
Also, when you get away from Southern Sonoma and Napa, the tasting are more personable and sometimes even free. Everything is more down to earth. Big names and very wealthy people own wineries in Napa and Southern Sonoma; in Northern Sonoma, Mendocino and Lake, some very ordinary people have spent their life savings and started their own vineyards and wineries. One we really liked (and will return to) was owned by a retired firemen.
One place in Mendocino (Golden Eye I believe) on highway 128, they sat us down in front of a nice fireplace, brought out 6 different Pinots, already poured; and some food parings. The wine was old and excellent. The cost for the tasting? Nothing! Of course if you purchased a bottle, it would set you back about $40 or more.
The year before we went to the Central Valley to Paso Robles; then to just south of San Luis Obispo (cooler Pinot Noir country). We had a great time and tasted some nice Cabs, Merlot, Zins, Pinot Noirs and blends. We stayed at Avila Beach, which was one of the best places we have ever stayed.
These were all different experiences. I would go back to any of them. So don't be afraid to go to any of them. Just be sure to do your homework, so you know ahead of time where and what to visit.