Since everybody keeps mentioning it:

...whoa, this just fucking levelled me...

I think he did some great covers. Rusty cage, personal Jesus, Hurt, ++

I prefer the rougher material from his own music. With some nice stories in the lyrics. Cocaine blues, boy named Sue, solitary man, San Quentin, Folsom brison blues, ++

The man had a powerful voice, and led a damned interesting life. I'm not a big country fan, but classic artist like Johnny Cash not only transcend genre, but are timeless as well.

Yes, 'Hurt' on his last album. The most soulful l ever heard him sing. I saw him in a small bar in Louisville in 1967. He had just come out of rehab, and looking frail, but he was still terrific.

Use to be ...
Watch "Johnny Cash - Sunday Morning Coming Down" on YouTube

Hurt even though its not by him is my favorite.

I play guitar in a rockabilly band. We do Folsom Prison, Cocaine Blues, Ring of Fire, and Stripes.
I love a lot of his work, so to throw a slightly less common choice, i'll say "The Highwayman".

Ok, not a huge fan of the genre, HOWEVER, JC has some cool, and very singable songs. I like to "sing" Karaoke and think I do a decent job with some of his songs, like Folsom Prison Blues and Orange Blossom Special. Enjoyed Joaquin Phoenix in his film as well. =]

Hurt

I used to play "Ring of Fire" over and over on the jukebox at an old dive bar I went to in Detroit, but I think his version of "Hurt" is my fave. It's beautiful but painful and just rips your heart to shreds.

Hurt

Sunday Morning Sidewalk is my favorite Cash track. But there are so many classics credited to the Johnny Cash library.

Even though it wasn't written by Johnny, the song 'Hurt' is hands down my favorite. Trent Reznor wrote and performed it, but when he heard Johnny sing it, he told the Cash man he could have the song.

Not a big U2 fan but:

What is Truth

I love Johnny Cash. To me, this is one of his most touching performances - and speaks to the events of today as it did on the day he first performed it. Listen to his introduction, and feel the hurt, the anger, and the empathy that comes from this song. This is an unpolished and raw initial performance of the Man in Black.

I have a weird fondness for Melva's Wine. An obscure old track my Mum had on cassette, that I used to listen to again and again when I was maybe 10. It's a sweet story.

There are many songs that I like by JC who was an American icon!

I grew up with Johnny Cash music all around me and he was so popular with me that I played his songs sometimes even during my last marriage. My Kenya ex-wife learned to identify Johnny Cash music in the first 5 musical notes of a Cash song.
Overall our music interests were varied and still are.

Almost anything I hear Johnny do is good to great. Jackson, Ring of Fire and the list goes on forever. I sometimes put You-Tube videos of his concerts and they play all day long. The album - Mean as Hell is little known I think but fantastic.

I walk teh line
Ring of fire
If I were a carpenter
Jackson

Rusty cage is good, too.

Delia, oh Delia, my favorite Cash song. He did so many religious tunes that it can be difficult finding the good secular ones.

Absolutely