Which of these reliably red deep south states has the most potential to become more competitive?

[Del Tachi]

We can almost immediately cross Alabama, Mississippi, and Tennessee off the list. 

Though Alabama elected a Democratic Senator about two-and-a-half years ago and Tennessee's native son drew within four points of George W. Bush in the 2000 Presidential Election, these states are just too far inelastic to become competitive for the blues within the near future (at the Presidential level, anyway). 

My pick would be South Carolina, but it would be a personal toss-up between that and Louisiana. 

In order for any Democrat to make any of these states competitive, they would have to run a charismatic southerner with JBE-type politics against a weak (and potentially scandalized) Republican opponent.  And even then, I'm not sure that would be enough to outright win. 

Mississippi has hyper-racialized partisan voting habits (moreso than SC or LA, probably) but also a larger African American electorate than any other state.  MS is on-track to being plurality Black as early as 2040.  That alone makes its the most likely state in the Deep South to eventually become competitive.