If you had to choose just ONE.

[Dave B]

The most beautiful classical music piece you've ever heard.
If you can name just one that stands out. I know our inclination is to list 50 immediately.
Mine is Handel, Concerto Grosso Op.6 No 6, in G Minor.
III. Musette: Larghetto.
I may have asked this years ago when I was on before. 2008 or so.

[Symphonic Addict]

Sibelius' Seventh Symphony would be my candidate. This piece contains music of ineffable beauty and purity that moves me enormously. I feel it has to do with something about the power of nature, the spirit of Earth speaking through Sibelius that make it so special and incredibly majestic. Oddly enough I didn't appreciate this symphony when I started listening to it, it was later when it clicked on me.

[(poco) Sforzando]

The love scene from Berlioz's Romeo and Juliet. Munch or Bernstein.

[Holden]

My opinion might change next week (in fact it probably will) but at the moment what comes to mind is the slow movement of Ravel's G major Piano Concerto.

[JBS]

First thing to come to mind that's not the Bach Chaconne is the Andante ma moderato (second movement) of Brahms String Sextet Number 1 in B Flat Opus 18.

And I seem to be not the only person who likes it

This sextet was used as soundtrack by French director Louis Malle in the movie "The Lovers" ("Les Amants", 1958). The sextet's second movement is featured in the Star Trek: The Next Generation episode "Sarek". The second movement is also featured in "The Day of the Devil", an episode of Inspector Morse, and in the 2001 French-Austrian film The Piano Teacher.

(From Wikipedia)

[Florestan]

Jacques Offenbach - Belle nuit, ô nuit d'amour (Barcarolle) from Les Contes d'Hoffmann

[Lisztianwagner]

The Love Duet from Wagner's Tristan und Isolde.

[Biffo]

Beethoven - String Quartet No 15 in A minor, Op 132 - Third Movement 'Heiliger Dankgesang'

[DaveF]

For sheer beauty of sound: Du Fay, Les douleurs, dont me sens tel somme.

[Karl Henning]

I know our inclination is to list 50 immediately.

Partly because settling on only one is ... a challenge. But, in the spirit of your query:

[Atriod]

This is likely to change tomorrow but I might as well include one of the earliest pieces that really blew me away in its beauty- JS Bach's Chaconne from the second partita. And the performance that did it was hearing my best friend's mother play it, I didn't know the terms for it then but upon hearing her play it many other times she plays without any vibrato but a very sweet portamento. Recording choice Giuliano Carmignola.

[DavidW]

The first movement of Sibelius' 6th symphony.

[Karl Henning]

The first movement of Sibelius' 6th symphony.

Oh, I approve. I keep wanting to add another, but I also appreciate that restraint was part of the exercise. 

[steve ridgway]

Ligeti: Lux Aeterna

[San Antone]

Duruflé - Requiem, especially the Pie Jesu movement as sung by Janet Baker.

[brewski]

Probably Richard Strauss Four Last Songs. Grew up hearing Gundula Janowitz with Karajan and Berlin, and still like that version, but since then there have been so many beautiful voices—and orchestras, since the instrumental role is crucial—that I don't have a clear favorite interpretation.

But for the original question, happy with that choice. PS, some lovely selections in this thread.

-Bruce 

[Keemun]

Bruckner: Symphony No. 7, II. Adagio.  Some of the passages are incredibly beautiful, in my opinion.  The recording by Karajan and the Vienna Philharmonic is the best I've heard of this symphony, and that movement in particular. 

[kyjo]

First thing to come to mind that's not the Bach Chaconne is the Andante ma moderato (second movement) of Brahms String Sextet Number 1 in B Flat Opus 18.

Great choice! Sometimes I find that Brahms' slow movements are the weakest part of his compositions, but certainly not so with the magisterially powerful slow movement of the 1st String Sextet. In fact, the whole work is ineffably beautiful, undoubtedly one of Brahms' finest achievements IMO.

[Spotted Horses]

Great choice! Sometimes I find that Brahms' slow movements are the weakest part of his compositions

Why do we have a "like" button but no "loath" button! 

[LKB]

Duruflé - Requiem, especially the Pie Jesu movement as sung by Janet Baker.

If you're referring to the EMI recording of the organ version with Ledger & Co., I would most definitely support your choice - it's the only version l've ever owned. Between the atmospheric recording, tonal beauty of Ledger's King's College Choir and the pleading radiance of Dame Janet ( plus the Cello soloist whose name I've regretfully forgotten ), the recording approaches ( imho ) perfection.