Dunno if there's one "saddest" or one "happiest," but in addition to some that have been nominated here, I'd add:

Really sad:  "Un Canadien Errant"
Really, really sad:  "Empty Chairs and Empty Tables" (from "Les Miserables")

Really happy:  "Oh, What A Beautiful Morning!"
Also really happy:  "Chelsea Morning"  (Joni Mitchell)

And one that just plain makes me feel good:  "In Spite Of Ourselves" (John Prine)
 

BTW, Barnacle, I thought that line was from "Maude Mullins."  "The saddest words of tongue or pen, The saddest of these: "It might have been."  (Whittier?  Wordsworth?  I can't remember for sure.)