The biggest is not even used as food, over half of corn acreage is used for ethanol. That's an amount of land that's truly beyond comprehension. Its a horrible program as well, corn ethanol is worse than the gasoline it replaces in terms of carbon footprint when taking land use into account. And it raises the price of food. And we even subsidize it multiple times, we subsidize the crop as corn and then we subsidize it as ethanol. Biodiesel and renewable diesel (different products) have spiked in recent years as well, most of that is made from soy, canola, or corn oil. They have similar problems though aren't as bad as corn ethanol.
Another huge negative surplus is the amount of liquid calories, mainly soda, that are consumed. Most nutrition science that I've read points to the enormous amount of liquid calories as the part of the US diet that is driving obesity epidemic. There are of course other aspects to the obesity as well.
Finally, substituting some of the US consumption of beef with chicken and some of the chicken with beans.
To recap US overproduces calories to the point that it hurts the country. It damages the land, the ocean with dead zones, the climate with carbon. We pay for it multiple times in subsidies and with higher food prices. It hurts our health which we pay for in suffering, shortened lives and health costs.