Apple pie

According to a pie chart (seriously) from the American Pie Council, apple really is the U.S.'s national favorite -- followed by pumpkin, chocolate, lemon meringue and cherry.

Not to burst the patriotic bubble, but it's not an American food of indigenous origin.
Food critic John Mariani dates the appearance of apple pies in the United States to 1780, long after they were popular in England. Apples aren't even native to the continent; the Pilgrims brought seeds.
So what's the deal with the star-spangled association? The pie council's John Lehndorff explains: "When you say that something is 'as American as apple pie,' what you're really saying is that the item came to this country from elsewhere and was transformed into a distinctly American experience."
And you're saying Americans know something good enough to be an icon when we eat it, with or without the cheddar cheese or vanilla ice cream on top.