It's impossible to pick a favorite cut but I've always thought she had a particular genius for fitting perfectly into the music of others without ever becoming a 'background' singer. Her "Duets" album is an amazing showcase of that talent.
I'm particularly fond of "Love Hurts" with Gram Parsons, "That Lovin' You Feelin' Again" with Roy Orbison, "We Believe in Happy Endings" with Earl Thomas Conley and, best of all, "Gulf Coast Highway" with Willie Nelson.
She even does a commendable job of making John Denver sound pretty good on "Wild Montana Skies" but I'll have to admit I made a copy of the CD with that one deleted.
-