How often does a cover truly equal an original performance, let alone surpass it? Not that often, to my ears.
I can respect that point of view, @stuartk , and to some extent, although I find covers of songs I am familiar with to be interesting, I share it. For me the notable exceptions include The Cowboy Junkies cover of Sweet Jane (and possibly Mr. State Trooper) and also Merle Haggard and Willie Nelson’s cover of Pancho And Lefty (you already mentioned Emmylou, and she also did a cover of that also that is simply beautiful). I’d probably add Lucinda’s cover of Return Of The Grievous Angel. I have read that many, including Bob Dylan himself, prefer the Jimmi Hendrix cover of All Along The Watchtower, but I don’t feel that way about that one. On a quick edit, I think that I might prefer Sinead O'Connor's cover of All Apologies to the original.
As far as Linda Ronstadt, I bought the Simple Dreams 8-track shortly after I had graduated high school, and since radio stations were severely limited where I grew up, I grew up actually thinking that she was the original artist on most of those songs. But now that my musical horizons have been expanded, I think that I still prefer her interpretations on many of the tracks. Poor Poor Pitiful Me and Carmelita and Blue Bayou come to my mind immediately.