Walmart is the largest grocery retailer in the world, with thousands of locations. As perhaps the most widely available grocery chain, Walmart caters to diverse populations across the U.S. So, if a product is a best seller at Walmart, the masses have probably approved of its quality. And when it comes to boxed brownie mixes, Walmart customers have spoken. The best-selling boxed brownie mix at Walmart isn't Pillsbury or Betty Crocker, but instead, is Duncan Hines.