Of all the songs I've heard Johnny Cash do I think my favourites have to be:
"A Boy Named Sue"- it's catchy, funny and shows his quirky side
"Folsom Prison Blues" - cannot believe he wrote so powerfully without having the personal experience at the time.
"Big John" - just a good ballad - corny in parts but GOOD.
"I Walk the Line" - moving

and

My absolute favourite:

"Man in Black" - says it all

BTW - I think his biography is also called "Man In Black"

To wax (maybe) controversial; I felt Joaquim Phoenix played him brilliantly in the biopic "Walk the Line" and Reese Witherspoon as June Carter was also superb. I also felt the film portrayed him fairly honestly too - warts and all - no excuses, just - as it was.

Muttley

Haven't heard some of the others mentioned: didn't grow up un a folky family or one that appreciated Johnny Cash - but I loved his songs when they came on the radio over here (Oz) (which was, unfortunately, an infrequent occurrence.