The Del McCoury Band with special guest Karl Marx (actually, it's David Grisman on mandolin, but the resemblance is uncanny). It's a fairly simple three chord song, but the improvisation - especially in the mandolin duel with Grisman and Ronnie McCoury - goes down some really cool rabbit holes. Also, on guitar, is some classic stuff from the guy who gave a whole new flavor to bluegrass music with his signature ending of the standard Lester Flatt "G run" on an F note.