(recipe, Mark Bittman)
Ground chuck — which generally corresponds to the "20 percent fat" ground meat sold in supermarkets — makes the best burgers; the fat acts to keep it moist. An overcooked but fatty burger will still be moist; a lean burger will be unappetizingly dry.
Given concerns about the safety of store-ground meat (E. coli, salmonella, and the like), you might want to try grinding your own meat for burgers. Buy a chuck roast, cut it into small cubes about an inch square, and pulse a small batch (about 1/2 pound) at a time in a food processor. Make sure you don't pulverize the meat and it'll be wonderful. Freeze what you don't use immediately. If you do buy pre-ground meat, most authorities recommend that you cook it to well done (160 degrees) to kill harmful bacteria. Do this once and you're likely to begin grinding your own meat, since well-done hamburgers are often too dry to eat.