Toru Takemitsu: Dorian Horizon for string orchestra
Messiaen: Turangalila / Trois petites liturgies pour la presence divine / vingt regards sur l'enfant Jesus, many other piano works and organ pieces / variations for violin and piano.
Ingram Marshal: Fog Tropes (original version with prepared tape and brass, II is the same prepared tape and string quartet)
Kevin Volans: Concerto for piano and winds.
Robert Moran: Chant du Cygne, Requiem
Ives: Three places in New England / Symphony No. 4 (imho, a masterpiece) / many songs, the orchestral 'sets.'
John Adams: Shaker loops / Hallelujah Junction / Century Rolls / Common Tones in Simple Time / Dharma at Big Sur
Stravinsky: Apollo / Concerto in D for string orchestra / The Rake's progress / Les Noces / Renard / Concerto for two solo pianos / Capriccio for piano and orchestra / Threni / Requiem Canticles
William Schuman: Some of the symphonies, sadly neglected and underexposed.
Milhaud: Six Petites Symphonies / Les Chorephores / La Creation du Monde / La muse Menagere. Viola Concerto
Carl Neilson: Symphony No. 5
Hindemith: Kammermusik, most of the first six, especially no. 1 and 4 - the latter a concertante piece for violin and ensemble / Mathis der Mahler
Honegger: Pastorale d'ete / Concertino for piano and orchestra
Berio, a number of works, Visage is one (I can't recall all titles now) other than but including the earlier Sinfonia.
Morton Feldman: Triadic memories / Crippled Symmetry / Piano and String Quartet.
Witold Lutosławski: Piano Concerto / Symphony No. 3
Lucia Dlugoszewski: Fire Fragile Flight
Debussy: La Mer
Bartok: Cantata Profana
A lot of Prokovieff
Stockhausen: Gesang der Jünglinge
Barber: Capricorn Concerto / Piano Concerto / Summer Music
Boulez: his early cantata - Soliel des Yeux
Elliott Carter: Piano Sonata / Cello Sonata / Sonata for Flute, Oboe, Cello and Harpsichord / Double Concerto for piano, harpsichord and two chamber orchestras / A symphony of three orchestras / A Mirror on Which to Dwell for Soprano and Ensemble / Syringa for Mezzo-Soprano, Bass-Baritone, Guitar and Ensemble.
Schoenberg: Kammersinfonie op.9
Berg: Lulu / Violin concerto
Webern, amongst a good handful, the Cello Sonata.
This list is partial and by no means near complete (if I gave it more thought and time than immediate memory while on-line, it would be overly long).
I've left off other already listed pieces, ex. Petruchka; Cage, piano concerto 'the seasons' etc.
Best regards.