My 2 cents, fwiw.
Cobain was a genius. A sick genius, but a genius none the less. If you listen to his lyrics they are pretty guteral and sick. I think he killed himself because he LIVED that stuff.
Cash was okay but not my cup of tea. He was an icon, much like Elvis, who also didn't write his own music. It isn't music critics who decide who is great, it is the people. The people loved Cash and Elvis so who are the "critics" to decide if they were worthy or not.
Neil Young WAS one of my favorites of all time. Learned to play from his songs. His liberalism turns me off but he isn't vehemenent about his POV and has always understood that of others. Plus, he still makes good music--it just doens't get played on the radio anymore.
Bruce Springsteen? I'll never understand how he even got a record deal. His only talent was being "The Boss" and leading a capable band through working man's anthems.