I'm torn between the double cheeseburger (BK), the McDouble, and the Jr. Cheeseburger Delux. But of course, there are many options.
http://www.mcdonalds.com/us/en/food/meal_bundles/dollar_menu.html
http://www.bk.com/en/us/menu-nutrition/lunch-and-dinner-menu-202/index.html