The single most beautiful piece of music is...

The second movement, Largo, from Symphony No. 9, From the New World, Antonín Dvořák, 1893.

I admit it's a conventional choice. Why do I think so?

• It lifts my spirit although I've heard it hundreds of times and still listen to it nearly every week.
• In the space of only 12 minutes, it showcases almost every major aspect of classical music: changes in tempo, changes in dynamics, brass, woodwinds, strings (including a bass pizzicato), just enough percussion, solos, a trio, happy moments, delicate moments, somber moments, quiet moments, a comprehensible structure, and the harmonies of the opening four bars.
• I clearly remember hearing it for the first time -- on an FM radio.
• Without this piece, could there have been Aaron Copland?

Other nominations are welcome, but I've set the bar very high. And by the way, it's a coincidence that I resemble the New York Philharmonic violinist (Kenneth Gordon, now retired?) in the fourth row.