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.)