What is the most beautiful piece of classical music you have ever heard?

There is no definite answer for the most beautiful piece of classical music ever heard with the answer depending on the person you ask.
Different types and styles of music appeal to different people although some pieces such as Luciano Pavarotti's Nessun Dorma, made famous at the 1990 Soccer World Cup in Italy, is likely to feature highly in any list compiled of the most beautiful pieces of classical music.
Even amongst classical music enthusiasts there are pieces from composers such as Johann Sebastian Bach, Ludwig van Beethoven and Johannes Brahms that are much loved and strongly debated as to their merits.
The endearing features of classical music sees those pieces stand up to the test of time, retaining their appeal and magic for different generations who discover and appreciate the qualities of these pieces.
Each of those composers, and others, will have pieces that fans believe to be the best ever written but in the end debates like these come down to individual preferences.
At different times, different pieces may gain in popularity but the beauty of classical music is that it can inspire debate with equally strong cases being put forward for various pieces.
Most of the best-loved pieces are centuries old and became popular in an era when modern communication was almost non-existent, the only people to be aware of the music had heard it live in performance and taken it to heart.
Down the years classical music has broadened its appeal beyond the elite in society to become truly popular and inspire some of the most popular song and music writers of the late twentieth century.
With music much more accessible these days, the ongoing debates about the finest classical music pieces is set to continue with no right or wrong answers.

Pachelbel does not even compare to the beauty of anyone of these:

Rachmaninov Piano Concerto #2 in C Minor- 2nd Movement Adagio Sostenuto

Elgar: Enigma Variations. Especially the 9th Variation (Nimrod). The whole piece is an autobiography of himself, and the Nimrod variation is a dedication to Elgar's deceased best friend.

Barber: Adagio For Strings, and also Barber's Violin Concerto (1st and 2nd Movements especially)

Dvorak: Symphony #9 (New World). Dvorak, a czech composer, lived in Iowa towards the end of his life and the masterpiece so beautifully describes the endless fields of Iowa, the feeling of renewal and hope (especially for a man nearing the end of his life). So beautiful.

Brahms: Symphony #1. Often called Beethoven's 10th Symphony, but this piece is so moving and powerful. The last two movements are very much in the Romantic Era. So detailed, but so simple.

I love Canon In D. I play piano, and everything I play it *I know this sounds corny, but bear with me* , I can feel it inside of me. Fur Elise is another one that makes me feel wonderful.

Clair de Lune, Arabesque no. 1, op. 10 no. 3 (Tristesse), Sonata pathetique mov. 2, op. 9 no. 2, rev ire, many many more.

BUT THOSE ABOVE FAIL IN COMPARISON TO "ADAGIO FOR STRINGS"!!!  It's amazing! It leaves me completely breathless.... If you like it, "Agnus Dei" is just as amazing! It's choral. Samuel barber wrote adagio for strings then transposed it it into a choral piece.

Four Seasons; Spring - Vivaldi

Nocturne op. 27, no. 2 by Chopin.
Etude op. 10, no. 1 by Chopin.

Adagio for strings or the second movement of beethoven's fifth. Anything by Debussy is a sure fire relaxer as well.

Beethoven No. 7, 2nd movement, Allegretto. Often overlooked symphony - didn't see it mentioned anywhere else. But to me this is a perfect composition - lots of drama and incredible build-up.

Canon in D is the only piece that will never die. It can beat all! Pachelbel isn't the best composer but his Canon in D is absolutely perfect. No doubt, it's the most wonderful piece of music ever composed on the Earth.

Vivaldi's four seasons or Pachelbel's Canon in D major

Claire de lune by claude debussy its a fantastic song the best music I have ever listened to

Definitely Mozart's Clarinet Concerto (Adagio) in C minor. If I were to define the word "beautiful" I would say: 'A thing that pertains to the qualities of aesthetic splendor/Mozart's Clarinet Concerto.

I love Bach - Cello Suite No. 1 in G Major or Beethoven Moonlight Sonata. My two favorite songs to relax me.

Mozart, piano concerto no. 24 in C minor, K. 491! It's way less famous than it should be.

Schwanengesang D 957: No 4 (Swan Song) by Franz Schubert. You will almost certainly never hear a more beautiful piece.

Beethoven's 5th and especially 7th Symphonies, Gershwin's Rhapsody in Blue, Mozart's Requiem, Orff's Carmina Burana, Bernstein's Mass

Mozart Piano Concerto number 21, "Elvira Madigan". Brendel has produced a superb double CD containing this and other great piano concertos -- my favourite CD. Mozart's Clarinet Concerto comes close!

I'm surprised no one mention puccini's (sp?) intermezzo from the 3rd act of his opera Manon Lescaut; it builds so much tention in such a short time. And I like the person who said that its a piece unless it has words my band director always stresses that it makes sense

Samuel Berber's Adagio for Strings.

Beethoven's Piano Concerto No. 5 2nd Movement

Bach's mass in b minor

Music from "The Balcony Scene" from Romeo and Juliet

Okay, I'm going to be arrogant...most of you are daft! 

I sample your nominees and many of them are brash and noisy, not soft and gentle at all. "Most beautiful" to me means it touches your heart, beauty that makes you start to cry tears of joy and sadness simultaneously. A sound so beautiful it causes you to think of loved ones, your mortality, the innocence of  childhood, the beauty in the world.

That piece, the most beautiful 'song' in the world is without a doubt Pie Jesu. And it's Andrew Lloyd Webber's version.

BOOM.  Mic drop.

End of story.

;-p

Bach  - St. Matthew Passion is without doubt THE most beautiful piece of classical music I've ever heard

Bach Cello suite no1
Mozart's Air
Chopin op9 no2
If God comes to earth, we have these to show him.

That question will never have a RIGHT answer for you or anyone. Many people will agree with 'Love Love' but, most will pick something else. Love to you.

Personally, Ja Rule's tempo in "put it on me" is exeptional...Next in line is Sisco with "The Thong Song"

Fantasia on a theme by Thomas Tallis (Vaughn-Williams)
Dvorak's Slavonic Dances and symphonies #8 and #9
Agnus Dei (choral version of Adagio for strings by Barber)
Too many more to list, but those are my personal favorites

1. Mozart Requiem - Lacrimosa
2. Mozart Ave verum corpus
3. Rachmaninov piano concerto nr 2

A "song" generally has words - a piece is without words.

Personally, "Our Town" by Aaron Copland
The piece is moody, deep, filled with plot lines deeply entrenching, and it overall has that Copland feeling of very ethereal type big body work which really make you appreciate the beauty of the world outside.

The best music I have ever heard is isle of Skye it's by Doris Gazda he made it a very long time ago in fact I'm playing that song in my concert which I'm in middle school.

Johann Pachelbel's Canon in D, it' a fantastic song and reminds compostion. It gives me chills everytime I hear it...it totally reminds me of my wedding day.