San Antonio over all of them. You can visit Pee-Wee's bicycle in the basement of the Alamo.
All kidding aside, San Antonio + Austin = 5+ million people within driving distance. No other site has numbers like that.
Ranking by potential media market
San Antonio + Austin (similar to Phoenix, Atlanta)
Charlotte, Portland, Nashville (St Louis, Pittsburgh, San Diego)
Salt Lake City (Kansas City, Milwaukee, Cincinnati)
I doubt MLB will want to bring in another revenue sharing recipient in SLC. Charlotte and Nashville would be revenue neutral. Charlotte would capture all of North Carolina's attention (11M people) and Nashville would get all of Tennessee (7.3M). Those numbers are significantly higher than Oregon (4.3M) or Utah (3.5M).