Probably my all-time favorite is Jule Styne & Sammy Cahn's "I Guess I'll Hang My Tears Out To Dry" (from the movie "Glad To See You". And Jule Styne came up with a few other great "unrequited love" songs too, such as: 
"The Party's Over" ("Bells Are Ringing") 
"The Music That Makes Me Dance" ("Funny Girl") 
"Not Mine" ("Hallelujah Baby") 
 
Some other great ones that spring to mind are: 
Jerry Herman's "Time Heals Everything" ("Mack and Mabel") 
Charles Strouse & Alan Jay Lerner's "There's Always One You Can't Forget" ("Dance A Little Closer") 
 
		     						     						
Another Rodgers & Hart... "Glad to Be Unhappy" from On Your Toes.
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