If you had to choose just ONE.

[Wanderer]

Today, that would be a choice among Beethoven's Piano Concerto No. 4, Mozart's Così fan tutte, Puccini's La fanciulla del West, Korngold's Das Wunder der Heliane, Zemlinsky's Eine florentinische Tragödie  and Schreker's Die Gezeichneten.

[classicalgeek]

How can I name just one? But I'll certainly try... for me it changes regularly, but as of May 22, 2024 it's got to be Mahler, Symphony no. 4, third movement.

honorable mentions:
Dvorak, Slavonic Dance, op. 46 no. 4, the opening.
Bruch, Scottish Fantasy, the first movement proper.
Brahms, Alto Rhapsody, after the switch to C major ('ist auf deinem Psalter')
Chopin, Nocturne in D-flat, op. 27 no. 2

There are SO many more, especially when sections of movements or pieces are factored in. I guess that's what works about Mahler 4:III - it's breathtakingly beautiful, start to finish!