Wait, I thought by "fast food" you meant the actual kind of fast food (fries, onion rings) and not the restaurants (McDonald's) and/or cultures (Chinese) that may serve the food...
For me, it's fried chicken. So that would be KFC and basically every other fried chicken restaurant out there. I don't know what it is about it, but I guess it's just so fatty and bland [once you get to the meat of the chicken piece].