I've tried to spread out my twenty-five choices over the centuries.

Here goes:

1.  Josquin: Missa Pange lingua
2.  Morley, et al.: The Triumphs of Oriana
3.  Byrd & Tallis: Cantiones sacrae
4.  Purcell: Dido and Aeneas
5.  Monteverdi: Vespers 1610
6.  Bach: Well-Tempered Clavier I+II
7.  Bach: Magnificat
8.  Handel: Messiah (or Israel in Egypt)
9.  Handel: Concerti Grossi, opp. 3 & 6
10.  Haydn: Sinfonia Concertante
11.  Mozart: Symphony No. 39
12.  Mozart: String Quintet in g
13.  Mozart: Don Giovanni
14.  Beethoven: Symphonies 1-9
15.  Beethoven: Violin Concerto
16.  Beethoven: Piano Concerto No. 4
17.  Schubert: Cello Quintet in C
18.  Brahms: Variations on a Theme by Haydn
19.  Wagner: Der Ring der Nibelungen
20.  Mahler: Symphonies 1-10
21.  Stravinsky: Le Sacre du printemps
22.  Schoenberg: String Quartet No. 2
23.  Bartok: Music for Strings, Percussion, and Celesta
24.  Webern: Variations for Orchestra
25.  Debussy: Trois Nocturnes

These aren't necessarily my favorite works or even my favorite
composers, but I do think them representative and influential.  I tried
to balance genres as well as pick works that are outstanding as well as
representative.  One reason that many of my favorite composers don't show
up is that they are primarily individuals, rather than representatives or
particularly influential.  For example, I chose Stravinsky's Sacre rather
than a lot of other Stravinskies because it's quite simply the symbol of
modern music.  Schoenberg's String Quartet No. 2 I picked because it shows
2 1/2 styles of Schoenberg (it's a transitional work), so you can see
Schoenberg moving from late Romanticism to his freely atonal and even
serial style.  I ran out of slots before I could get to postwar music.
Still, some of my favorites did make it because I couldn't bear to leave
them out.  Beethoven's Piano Concerto No. 4 I doubt was influential in its
time - the Emperor seemed to predict the course of the concerto in the
short term - but it's such a marvelous piece.

Steve Schwartz