Hands down, the best Mexican food in Madison (though to be honest, there is very little decent Mexican good in this city). Usually I order delivery, though if you go to the small hole-in-the-wall restaurant, you'll be greeted by a very authentic experience; it appears to be run and frequented by Mexican-Americans. Just don't come looking for an upscale experience or a place with plenty of cocktail options or a nice wine list.
The Burrito Mexicano is delicious and comes looking like a Mexican flag on a plate.