The Costco Dog and Soda for $1.50 USED to be at the top of my list, but they ruined it when they dropped the Hebrew National dogs for the nearly inedible Kirkland ones - the Kirkland Polish is lousy, but it's an order of magnitude better than the Kirkland hot dog.
Nowadays, I'd have to say that the McDonalds McDouble and Sausage Breakfast Burrito on their dollar menu are the two top cheap eats. Although, on Wednesdays, The Burger King Whopper is worthy at the Whopper Wednesday price, but not at the regular price the rest of the week.
Pollo Loco Chicken Soft Tacos and Loco Salad are also competitors, IMHO, significantly better than the various Taco Bell offerings.