You might enjoy this too, Wolfstar :)
If we're talking classical symphonic music, I like Beethoven's 5th Symphony. I also like Massenet's 'Meditation' and Ravel's 'Bolero.'
If we're talking inspirational/religious music, my all-time favorite is Franz Schubert's 'Ava Maria.' I heard Pavarotti sing it once, and nothing quite touches my soul like a great performance of that song.
If we're talking patriotic music, no list of mine would be complete without the '1812 Overture,' 'Stars and Stripes Forever,' 'Taps,' 'God Bless America,' 'America the Beautiful,' and the 'Battle Hymn of the Republic.'
If we're talking musical theater, I love the 'Soliloquy' from Carousel.
And that's just for starters. If we're talking American folk and popular music, I love songs from every era going back to at least the mid-1800's.
How can anyone pick just ONE all-time greatest piece of music? A piece from the Middle Ages is still popular today because of its haunting beauty. We know it as Greensleeves. Its longevity, alone (over 400 years old), qualifies it as one of the greatest musical pieces of all time.