Well some, which come to mind for right now.

Messiaen – Quartet for the End of Time
Arvo Pärt – Tabula Rasa
Steve Reich - Drumming
Toru Takemitsu - Requim
Terry Riley - In C
Alexander Scriabin - Piano Music (Études, Preludes, Sonata #5)
Samuel Barber- Adagio for Strings Op. 11/2
Béla Bartok - Concerto for Orchestra Concertos for Piano ( 3-5)
Benjamin Britten - Serenade for Tenor, Horn and Strings Op. 31
Maurice Durufle - Requiem
Henryk Gorecki – Symphony No. 3
Paul Hindemith- Mathis der Maler (symphony)
Gustav Holst - The Planets (suite)
Leos JanáÄ?ek - Quartets 1 & 2
Bohuslav Martinu - Double Concerto for 2 String Orchestras, Piano and Timpani
Carl Nielsen - Symphonies (1-5)
Joaquín Rodrigo - Concierto de Aranjuez for Guitar and Orchestra
Arnold Schoenberg- Chamber Symphony No. 1
Heitor Villa-Lobos - Bachianas Brasileiras #4 & 5
Milton Babbitt - Piano Concerto
Pierre Boulez - Memoriale a Bruno Maderna
John Cage - Music for String Quartet
Morton Feldman - Rothko Chapel
Philip Glass - Low Symphony
Lou Harrison - Symphony No. 2
Ernst Krenek - String Quartets


Richard Strauss
Also Sprach Zarathustra Op. 30 (Symphonic Poem)
Four Last Songs for Soprano and Orchestra
Till Eulenspiegel Op. 28 (Symphonic Poem)

Igor Stravinsky
Firebird (ballet suite)
Petrouchka (ballet)
Sacre du printemps

Karlheinz Stockhausen
Klavierstücke
Kontakte
Mantra
Stimmung

Anton Webern
Music for String Quartet
Five Orchestral Pieces

Yannis Xenakis
Metastasis
Palimpsest

Aaron Copland
Appalachian Spring (suite or complete ballet)
Concerto for Piano
Fanfare for the Common Man

Georges Enesco
Roumanian Poem
Roumanian Rhapsodies

Charles Ives
Three Places In New England
New England Holidays Symphony

Gustav Mahler
Das Lied von der Erde
Symphonies ( 1-9)

Sergei Prokofieff
Classical Symphony
Peter and the Wolf
Sonatas for Piano (2, 6-8)
Symphony #5

Ottorino Respighi
Fountains of Rome
Pines of Rome

Ralph Vaughan Williams
Fantasia on a Theme by Tallis
Symphony No. 3

György Ligeti
Atmospheres
Horn Trio

Dmitri Shostakovich
Cello Concerto No. 1
String Quartet #8
Symphonies (1, 4-8, 10-12, 14)

Jean Sibelius
Concerto for Violin Op. 47
Finlandia Op. 26
Symphonies (1-7)