We live in the area, and enjoy staying at the hotels and being "tourists" in our own area sometimes. There are quite a few choices here, but I think Calistoga Ranch Creek Spa rooms are as good as it gets. I rarely think the top hotels are worth the ratings and the prices they charge, but this small hotel group, "Auberge Resorts" I think, (chain is not the correct term!) does it all right.
They have 3 properties in Napa Valley: Calistoga Ranch being the 6 star, Auberge du Soleil 5 star, and Solage a notch or two below, still very nice, all three priced accordingly. (We have not stayed at Solage, but did stop in once.) Stayed at Auberge du Soleil in Dec 07, and it was fabulous. But if you really want to splurge, stay at Calistoga Ranch in a one of the Spa Rooms on the creek. I am not sure whether we had an Oak Creek or Bay Forest Spa room, but we were on the "lower side" which is probably preferable. Two separate structures, so you do have walk outside, which is fine unless it is raining or freezing, but that is not too common here and even then not a big deal. It was breaking records with the cold and we didn't mind it at all...made the hot tub nicer! In between the two structures you have a large deck, so it is a very OUTSIDE living room experience if you want it to be. Hot tub on the deck with an see-through glassed in gas fireplace to the indoor LR. Most of the year this means you have a LR, BDRM, and a huge outdoor living room as well. Oak trees are your ceiling, and most are arranged so you have fairly private space even when outdoors. Some even have trees growing THROUGH the decking. Some do face other units through the trees more than others, but you feel very private. Only problem might be you would never leave your deck to go visit the wineries or great restaurants! So go to a couple wineries and bring some back in case you stay on the deck by the fire the rest of your stay!
I must say that Calistoga Ranch is so secluded feeling, that you might prefer Auberge Du Soleil if you want to actually SEE many other people. Non-guests can't use the restaurant or spa here, so I doubt it is ever really hopping, but I have only been there 2 times. Non-guests can use the restaurant at Auberge, so it tends to be busier, if you prefer more activity. Spa was very very good, gorgeous setting in the trees also.
I knew it was the right choice when I called the hotel reservations desk and got a VERY delightful woman. She was very professional and yet super friendly at the same time, which can be difficult to do successfully. She told me her Brother in Law was a bartender there, and would be working during our stay, and to go to the bar to say "hi". We didn't, but appreciated the effort. That kind of chat could be unprofessional if done inappropriately, or with the wrong client, but she read me well and it was the perfect touch. It was the start of a perfect stay. From the cart drivers to the front desk to the spa, they all walked that line of warm yet professional. (Compared to Cavallo Point in Sausalito, where they were friendly but NOT professional.)
In short, if you want to splurge or just enjoy the best, stay here in a Creekside room. If your price range is the lower end of Calistoga Ranch, perhaps Auberge would be a better choice, as I think their starting rooms there are bigger than the starting rooms at Calistoga, but check them both out to see what your personal preference is. You can't go wrong in either place. Next time I will try Solage, and I look forward to trying their properties in the few other places they are!