I imagine there's few artists that have had a film that had as much impact on recognition of their music as "Walk the Line" did for Johnny Cash. It pretty much introduced a new generation to the music of Johnny Cash and boosted his popularity massively - even if the film didn't portray him entirely positively.
Personally I'd always known of Johnny Cash if I hadn't really listened to him much, my dad was a big fan and after seeing the film and hearing a bit more a raid was made on what Johnny Cash stuff he had and a compilation CD bought for a birthday and surreptitiously copied before being wrapped and handed over!
I'm not sure why Folsom Prison Blues sticks out so much for me, it is a favourite of my dads which is probably a good start. I think though it's in part due to the great "At Folsom Prison" recording which is portrayed so well for me in the "Walk the line" film. The track opens up the album and kicks off the performance well and is definitely the best version of the track to listen to.