New Orleans. But not only in South, but in all of US. Like Little Richard
, New Orleans is the originator of sooo much culture in the US....the music, the food, literature, diversity. New Orleans was the percusor to so much that is considered quintessential American culture. Before there was R&B, Rock&Roll, Pop, etc... there was Jazz. Even in diversity - unique culture originating from the French, African, Black, the Spanish, English soup of people.
Atlanta (like other places) have built upon New Orleans creations and may have surpassed in modern-day cultural impact. But if we want to be fair, the South in general has impacted American culture probably more than any other region. Yeah the cultural creations may have been re-fried, watered-down, gentrified and put on vinyl/film and commercialized in other regions but the originator ....