An obvious write-in for Basil Marceaux Dot Com.
Among those 4 though, it would have to be Gordon Ball. Despite his horribly fiscally conservative views (supports a flat tax, balanced budget amendment etc.), he is at least in the 21st century when it comes to civil rights.
As for the others, McWherter is a tax shifter for his wealthy friends, Brown is a huge NRA supporter and has arguably been running the worst campaign in the country this year (to even call it a campaign is probably an overstatement), and Clayton's a disgusting human being who for whatever reason thought he was running against Lamar Alexander in 2012. All completely unsupportable.