Southerners tend to go with a mustard based sauce, the upper Midwest with Ketchup based and southwest with molasses base. I like all three. The a brand out of Ohio - Montgomery Inn BBQ sauce, that’s really good.
Maurice’s Piggy Park, a small chain of BBQ restaurants in South Carolina, serves as their main sauce something that looks and tastes suspiciously like French’s yellow mustard.
I’m not sure that mustard based sauces are the predominant sauces outside South Carolina (although you can find them in more and more places). North Carolina used to be known for vinegar-based sauces.
Another BBQ where I’ve dined over the years is Golden Rule in Irondale, Alabama—I order the sliced pork plate, and this place has wonderful home-made French fries. The sauce in there is tomato-based.