Obviously there is no "best," only an infinite number of favorites and those may change depending on your circumstances. A couple of great ones that haven't been mentioned yet:

Stephen Foster's "I Dream of Jeanie" is still beautiful and stirring almost a century and a half after it was written.

"I Wouldn't Change You If I Could" Not sure of the title, or origins, I a bluegrass version by Reno & Smiley.