Here is my list of what I consider to be the essential Johnny Cash...
The Complete Sun Recordings 1955-1958 - As the title says, this is everything Johnny Cash recorded at Sun Records at the very beginning of his career. Most of the early classics are here, but there are several nice rarities and never-before-released songs. Most of the tracks feature the classic lineup of Cash, Luther Perkins and Marshall Grant.
The Fabulous Johnny Cash (1959) - Johnny's first major-label release (Columbia) after leaving Sun.
Now There Was a Song! (1960) - Johnny sings country standards. Includes an early version of "Cocaine Blues" called "Transfusion Blues."
Blood, Sweat and Tears (1963) - one of Johnny's first concept albums.
Bitter Tears (Ballad of the American Indian; 1964) - another concept album; includes the powerful "Ballad of Ira Hays," about the Pima Indian who helped raise the flag on Iwo Jima (as depicted in Flags Of Our Fathers).
Orange Blossom Special (1965) - a folk-country album, most notable for several Dylan covers, including the Johnny-June hit "It Ain't Me Babe."
At Folsom Prison (1968) - the legendary live album cut at Folsom.
Ragged Old Flag (1974) - notable as the first album in which Johnny wrote all the songs himself.
The American Series (1994-2006) - sometimes playful, sometimes dark, these five Rick Rubin-produced albums bring Johnny's career full circle. Many of the songs have a rough, ragged you-are-there feel, which was the intent. A great cap to an extraordinary career.
And finally...
Everybody Loves a Nut (1966) - although it's certainly not essential Cash, it's a fun album made up entirely of novelty songs, including the wonderful "The One on the Right Is on the Left," "Boa Constrictor," by Shel Silverstein, and "Dirty Old Egg Sucking Dog," which Johnny sang to Rowlf on The Muppet Show. Unfortunately, this is not available on CD. With The Carter Family and The Statler Brothers. The album cover is a drawing by famed cartoonist Jack Davis.