With a voice that crosses genres and has broad appeal, Johnny Cash is the Elvis of country music. Influenced by Blues and Gospel, and friends with the first generation of Rock and Rollers (occasionally even joining them, as with the famous Million Dollar Quartet).
Also, he wrote a novel. About the Apostle Paul.
Once of Cash's first hits, Cry Cry Cry is so steeped in Blues that Youtube has gone ahead and categorized it as a "Jazz and Blues" song.
On the other hand, his relation to Rock and Roll can be seen in his song Get Rhythm:
Another Cash's first solidly country-genre hit, as well as his first number 1, was I Walk the Line:
Cash released a long line of ballads, the most famous of which was his Don't Take Your Guns to Town:
Showing his depth and broad appeal, Cash crossed back into Rock and Roll and released Ring of Fire:
While there are far too many songs to list from Cash's extensive career, here are a few of my favorites that show how well he plays in numerous genres:
Tennessee Flat Top Box
There Ain't No Good Chain Gang (Duet with Waylon Jennings)
Ghost Riders in the Sky
Will the Circle Be Unbroken (With various country and bluegrass singers) Only the beginning is by Cash, but I think it shows that his voice goes fairly well with a group, even as varied a group as this one. These last songs also show his growing religious concern in his later years:
As a sequel to Charlie Daniels' hit The Devil Went Down to Georgia, Johnny Cash and other country stars produced The Devil Comes Back to Georgia
God's Gonna Cut You Down
The Man Comes Around