Plugra has a high fat content and is the favorite of many chefs but, to me, the cultured aspect of it comes off as sour. The Strauss Family butter sold at Whole Foods is what Thomas Keller serves at the French Laundry.
We like Kate's Homemade butter from Maine. Wild Oats carries it here north of Boston. It's made in small batches daily and has no preservative or additives - never frozen. It has a nice mild, fresh taste. We get both the unsalted and salted. http://www.kateshomemadebutter.com/