I’ve given this a great deal of thought, and I’m not sure why I’m bothering to start a thread on this, since the answer is unarguable, but:
The prettiest song ever is “Ripple” by the Grateful Dead (off American Beauty).
Glad to enlighten you.
Faust - It’s a Rainy Day, Sunshine Girl
The perfect track for a mildly depressive guy in love with a girl. It always brings a smile to my face. 
Shouldn’t that be the “pwettiest” song ever?
Anyway, my vote goes to “Caravan of Dreams” by Acoustic Alchemy.
The correct answer is: “The Man with the Child in His Eyes” by Kate Bush.
Second place goes to “Slow Tango,” by Jane Siberry.
Third place is “Lilac Wine,” as covered by Jeff Buckley.
“Danny Boy.” And I’m not even Irish!
Damn, Eve, I was expecting you to come in with something by Cole Porter – like “After You, Who,” perhaps.
Another illusion shattered.
Some candidates, speaking for the electronic world:
“How to Feed a Sea Monkey”, Land of the Loops
“Parallel Stripes”, Aphex Twin
“Corsair”, Boards of Canada
“Plateau”, The Orb
“Rachel’s Song”, Vangelis
…my idea of beautiful anyway.
“Wildwood Flower”
“Quand Vous Mourez de Nos Amours,” as sung by Rufus Wainwright
Failing that, “Complainte de la Butte”
To die for
“Stupid Mouth Shut” by Hem
She told me she loved me like a brother. She was from Arkansas, hence the Joy!
My immediate thought: “Here Comes the Sun,” George Harrison.
Prettiest music overall: Anything sung by Karan Casey.
It’s a tie between “Windy and Warm” by Doc & Merle Watson and “The Lady and the Unicorn” by John Renbourn.
Surely, twicks, there must be a scratch on your antique American Beauty LP. How else can you explain neglecting “Box of Rain” in favor of “Ripple?” 
I’m voting for:
Unforgetable
and
Nature Boy
Both as recorded by Nat King Cole
I’m voting for a good version of Debussy’s Claire De Lune played on the piano.
“Here Comes the Sun” – definitely a contender
John Renbourn – love that album, but prefer “Johnny Was a Carpenter”
“Somewhere over the Rainbow” as done by Eva Cassidy.
Anything Ella Fitzgerald ever put her vocal cords to.
Or, possibly, “Bonny Portmore,” an old Irish ditty, but it’s kind of sad and pretty, while at the same time being pretty sad.
Katie Sings, by the Minstrels of Mayhem. Just perfect.
“Consequence” by the Notwist, and lots of stuff by Sigur Ros. Also “Fresh Feeling” by the Eels.
How about Múm’s “Green Grass of Tunnel”?