When comparing musical compositions, keep in mind that it is impossible to compare apples with oranges. A Mahler Symphony just cannot be compared with a Bach Cantata - they are too different in style and scope, even though they can both be said to be complete masterworks.
Still, I think it would be possible for most people who listen to music regularly, to come up with a shortlist of works that are pleasant for them to listen to. Just one work, however, would not make sense for most serious music listeners. For me, there could be a list like this:
Dufay: Missa Ave Regina Coelorum
Josquin: Missa Pange Lingua
Monteverdi: Vespero Della Beata Vergine
Bach: Cantata BWV1, Wie schön leuchtet der Morgenstern
Haydn: Symphony #104
Mozart: Piano Concerto D minor K 466
Beethoven: String Quartet op. 131
Schubert: String Quintet in C major
Schumann: Kreisleriana
Bruckner, Symphony #8
Brahms: Violin Concerto
R. Strauss: Salome
Mahler: Symphony #6
Ravel: Piano Concerto in G
Debussy: Images
Stravinsky: Ebony concerto
Janacek: Sinfonietta
Bartok: Piano Concerto #3
Britten: Turn of the Screw
Shostakovich: Symphony #4
Stockhausen: Gesang der Jünglinge
Messiaen: Turangalila
Maxwell Davies: Eight Songs for a Mad King
Nørgård: Symphony #3
All these are works that I really like, and which give me an incentive to explore music of their composers, and then other composers further, though it would be impossible to rank them 1,2,3 etc. This does not mean that they are better works than so much other music, just that I like them.