1. Shostakovich - Symphony No. 5 - Shostakovich reformed. Shostakovich takes in Stalin's constructive criticism and produces a composition that denounces Stalin's regime while at the same proving Stalin correct that good music should be able to "enthral the masses". The work that simultaneously condemn and vindicate Stalin. This work is probably Stalin greatest contribution to mankind in his fight against artistic modernism.
2. Mahler - Symphony No. 9 - From Mahler personal level it represents his personal acceptance of his pending death due to bacterial endocartitis. However on a grander scheme it could be interpreted the pending symbolic death of tonality and hence music itself. This symphony also wraps up the symphonic repertoire quite well as well. Beethoven created the struggle and defiance against fate as the key to greatness, Mahler resolves the battle by saying in the end everyone must accept their fate as everyone must face death. In the way Mahler concluded Beethoven's symphonic cycle.
3. Mahler - Symphony No. 5 - The Beethoven Symphony No.5 of the 20th century and hence by default the greatest work of the century
4. Stravinsky - Pulcinella - Probably the only important work by Stravinsky as he demonstrated his original compositional techniques of rhythmic asymmetry, harmonic pedals, juxtaposition isn't completely mutually exclusive with the classical ideal. He briefly showed how 20th century could have developed creating original work without completely sucking. Unfortunately music didn't exactly go in this direction
5. Prokofiev - Symphony No. 1 "Classical" - Similar rationale to Pulcinella but ranked lower as I don't like this symphony as much as Pulcinella.
6. George Rochberg - String Quartet No. 3 - The symbolic nature of a ultra-serialist seeing the light and repenting his way by producing a tonal accessible work with the 3rd String Quartet. He realised through his son death that conclusion that he was unable to adequately express his profound grief and loss through serialism and hence realised the limitations of serialism in terms of expressive content. George Rochberg became a pariah that "I was accused of betraying, in the following order, the church and the state, I was a traitor, a renegade." with the work. While admittingly this is the case of too little and too late as the damage to classical music was already fatal it's nice to read a redemption story in music.
7. Gershwin - Rhapsody in Blues - Yes the music is compositionally sloppy (it has been described by Greenberg as the most technically deficient composition to be part of the music canon). However the incorporation of jazz into classical music by a popular songwriter in an crowd-pleasing accessible way deserves some merit in returning classical music back to earth as a popular art (in some way return back to the spirit of classicism) instead of the current reputation as an elitist art.
8. Glass - Violin Concerto - Some of the minimalistic compositions I've heard were probably even worst than the ultraserialist music it was rebelling against (at least ultra-serialist music can produce somewhat visceral negative reaction on the audience instead of the tedium boredom of the harmonic stasis of minimalism). However Glass Violin Concerto showed that the pulse nature and gradual transformation of minimalism when married with some degree of harmonic motion (even if it's basic I-IV-V type stuff) can produce music that is somewhat interesting to listen to.
9. Stravinsky - Firebird - I dislike the majority of this composition and I can't even call it a good work. However the section "Infernal Dance of the Ogre Kashchey" showed an innovative and thrilling "rhythmic development section" that could have form the basis of a uniquely Russian sonata form variant to the German model.
10. Gorecki - Symphony No. 3 - It takes a lot of chutzpah to perform this clearly tonal work in an avant-garde festival. I would have loved to been there and hear Pierre Boulez shouting out "merde" directed at the composer. Unfortunately this work is probably more important in that context rather than particulary enjoyable to actually listen to due to the harmonic static nature of the music.