This one appeals to my vanity. I would have loved to hear the late Kirsty MacColl do "Johnny Petto", because it straddles the worlds of folk music lore and punk rock - both of which she knew better than I do.
I would love to hear one of the great Irish folkies do "Black Crow Hill". I mean someone like Sean Cannon. He is a great singer and a nice bloke. I think he knows better than I do how an older bloke feels about the destruction of his community. I am probably not really old or experienced enough yet to handle that sort of material well.
There is the vanity side of this thread, but I think you also raise the question of, "Who understands the song best?" It is not necessarily the writer.