Yes indeed, not easy to pick just one. I think, for me , it will have to be "Chestnut Mare".    Shamefully I have never read a book about the Byrds (must rectify), but I was (and still am) intrigued
by the spoken interludes. It tells the same tale told by the leading character in Ibsen's play "Peer Gynt" (Act One). Did they borrow it fron Ibsen, or is there a common folk source ? Ibsen was well versed in folk tales.
Whatever, a great body of work, and very difficult to choose.