I know I'm not the only one who thinks McDonald's has the best fries out of most fast food chains. So what is their secret ingredient that makes McDonald's fries so addictive? Well, if you take a look into the ingredients, you'll notice that McDonald's has one key flavoring agent: natural beef flavor. The result is the elevation to a level of flavor that puts McDonald's fries out of the reach of its competitors in terms of deliciousness.