Their claim is legal, although it is a little misleading. The claim is based on the fact that they have sold more orders of fries than any other fast food chain. But you have to keep in mind that they have been open longer, and that it is the only side order they offer. Places like Wendy's have multiple side options, which means McDonald's ratio of sandwich to fry orders will always be higher than companies with more options.
I like Burger King's fries myself.
