Freemark Abbey might be one of your last stops on a wine tasting tour as it is north of St. Helena at the top of the Valley. Lucky for you, it is also open a bit later than some other tasting rooms (5:30pm I believe).
Their wines come from three different vineyards (Bosche, Sycamore and Red Barn Ranch). Although I have enjoyed just about every Freemark wine I have tasted, I tend to favor the Bosche Estate wines (Cabernet Bosche). They have nice Cabs and Viogniers - Freemark produces a many varietals (from Syrah to Sauvignon, Riesling to Chardonnay), so I recommend that you hit the tasting room to see which ones suit your palate.
Fun Fact: The winery sits on the site of the first winery to be opened and operated by a woman (Josephine Marlin Tychson, Tychson Cellars).