If Texas counts, then easily Texas. I doubt people in the rest of the world know much about Georgia, Mississippi, Tennessee, etc.
But then again, a lot of what Texas is famous for is its uniquely Texas qualities rather than its Southeastern culture.
This is one of the realest posts here. When people internationally think of Texas, they think of cowboys, bull ranching, steak, George W. Bush and the mantra that "everything" is bigger in Texas. NYC, Cali and Texas are probably the most notable states/cities internationally when considering the USA.