What is the most beautiful song ever?

Lots more--those are the ones that spring to mind right now.

Joni Mitchell: A Case Of You
Tom Waits: Martha
Yo La Tengo: Tears Are In Your Eyes (this is ridiculously beautiful)
Stevie Wonder: You And I
My Morning Jacket: Come Closer
Velvet Underground: Candy Says
Ron Sexsmith: Speaking With The Angel
Neil Finn: Try Whistling This
Flaming Lips: Do You Realise
Ben Folds: The Luckiest (my wife's choice)

But the all time number one winner that always gets me (even if a little cheesy) is:

The Hollies: He Ain't Heavy

Coz my sister sings it to me.

Definetely a contender, as is "Protection" (if Jed means Massive Attack)

Draft Morning - The Byrds

or

No train to Stockholm - Lee Hazlewood

but you really should try to find:

she moves through the fair- phil coulter

i'm also partial to the song during the baptism sequence of oh brother where art thou too.

Talk Talk - "After the Flood"
Olivia Tremor Control - "NYC-25"
Sonic Youth - "Karen Revisited"

I'll spare you the entire tracklist of Loveless.

The latter is great enough to redeem Sinead for whatever folk fault her for. The former could be the saddest song ever written, with a great central metaphor: "You made an impression, and sometimes I still feel the bruise."

Classical:
Messiaen - the cello/piano movement from Quatuor Pour le Fin de Temps (yeah alright, or the violin/piano one - I'm a cellist ok?)
Ravel - slow movement from the Piano Concerto in G (this is probably the most beautiful piece ever, but I may change my mind)
Mahler - take your pick from Symphonies 1, 4, 5, 6, 7...
Schubert - slow movement from E-flat piano trio, any movement from the Quintet for strings

Pop/rock:
Elvis Costello - Shipbuilding (HIS VERSION! Kills all the others, even Robert Wyatt's)
Belle & Sebastian - Don't Leave The Light On Baby
John Cale - Heartbreak Hotel, or (I Keep A) Close Watch (live from Fragments of a Rainy Season
Cocteau Twins - oh dammit, I can't decide... ella megalast burls forever is what I played on my birthday Utility Fog the other night, so maybe that? Does HTML markup work or did I fuck that up?
Leonard Cohen - Famous Blue Raincoat
Nick Drake - River Man / Fruit Tree
Elliott Smith - Waltz #2 (lots of other choices, but hey...)
Bobbie Gentry - Ode To Billie Joe
Talk Talk - The Rainbow/Eden/Desire, or your choice from Spirit of Eden or Laughing Stock...
Heligoland (astounding Tim Friese-Greene solo thing) - Bluebird
Machine Translations - Out To Sea
The Tall Dwarfs - The Slide
The Triffids - Tender Is The Night (The Long Fidelity)
Tom Waits - Cold Cold Ground / Yesterday Is Here
Mara Carlyle - Lost To Sea (etc)

I'm thinking about Tori Amos, the Clouds (or Jodi Phillis), and various other favourite artists, but... have they written really "beautiful" songs, as opposed to kick-ass, or moving, or...? I'm not sure.

Electronic/post-rock:
Aphex Twin - the suggestions above seem fine to me ;)
Autechre - eutow / that last track from Envane I think...
Boards of Canada - Everything You Do is a Balloon (this used to move me to tears, just like the Ravel above)
Dom & Roland - City (yes, it samples Badalamenti's City Of Lost Children soundtrack, but I think its beauty is down to the use within a drum'n'bass track)
Four Tet - And They All Look Broken Hearted (etc)
Fridge - Cut Up Piano And Xylophone
Funckarma - Lignite (from Part 5 12")
Gescom - Sciew Spoc
Global Communication - any choice from Pentamerous Metamorphosis, 76:14 (12:18 listed above may do the trick!)
Hood - too many to list! There must be one or two to pick out, but I can't think right now
Dntel - Suddenly Is Sooner Than You Think
Mia Doi Todd - Growing Pains (Dntel remix)
Plaid - Manyme (feat Mara Carlyle!) / Eyen
Squarepusher - Tundra
µ-Ziq (did that come out right? or:) µ-Ziq vs the Auteurs - Lenny Valentino 3 (1st track on the CD)
b.fleischmann - Take your time (esp last section)

Jazz/klezmer/gypsy
The Necks - Hanging Gardens / Mosquito (well I'm loving it right now)
Tin Hat Trio - anything from the last three albums? ;)
John Zorn - argh, not enough time to search through; one of the Masada String Trio ones from Bar Kokhba or Circle Maker probs
Kroke - Tzigoyner Tantz or... or...

...and I'm running out of steam. That'll do. Uh.

mahler 2 is really good!