It's hard to say what the most popular food is, but based on how many fast food restaurants there are in the country, the cheeseburger may possibly take the crown. Coming in close second would probably be pizza, then fried chicken and then finally tacos or burritos.
Pasta is the most popular food both in the US and worldwide. So many cultures use pastas for many of their dishes that Pasta is just a common item in all cultures. Pasta is a go to item in many cultures.