Buy the best you can find and afford. Look for a high butterfat content; the U.S. standard minimum is 80 percent fat. The better American and European butters are 84 to 87 percent fat. Ideally the butter is from cows raised on grass.
Unfortunately, many of the local, small-batch butters I find from grass farmers are a little gamey. Irish Gold is a great supermarket butter. Kate’s Homemade Butter, Vermont Butter and Cheese Creamery, and Straus Family Creamery make wonderful small batch American butters. Cultured butter is made by allowing a little natural lactic bacteria to ripen the cream. It’s tangy and delicious. In every dish except biscuits and hollandaise I use unsalted butter.