My favorite that I can get anywhere close to where I live is Arby's. I love the giant roast beef sandwich with Horsey sauce, and I love the Philly Beef&Swiss with Horsey sauce.
Favorite fast food I've encountered while travelling would probably be In-N-Out Burger. They have incredible made-to-order burgers with special sauce...half-wrapped in tissue paper to hold them together while you're eating them...MMMMM! Ultra-fresh fries that are cut a split second before being put in the oil are incredible too. Freshest fries anywhere.