I'd probably say real Bar B Que .Although a lot of cultures have sweet sauce dishes, I think American Bar B Que is great. It does vary region to region though, but that's mostly in the sauce (Some regions have molasses based, other vinegar etc etc)
Apple Pie. Maybe Potato Salad.
Like you said, America is so huge that food varies a ton between region. I went up to New England recently, and they have beans for breakfast! We also went to a restruant claiming their specialty was Key Lime Pie and Mojitos, and my family, being from coastal Florida, laughed at them. Other than that fluke I'd be hard pressed to find good Spanish, Cuban, and Tropical seafood (yes, it is different than new England seafood) up north, but I'm okay with that.
I think each region should just have their own mascot food.