But if we do stick to the 20th century, then the later music of Mahler also qualifies. Part of the 20th century canon should include the symphonies from No. 5 on as well as Das Lied von der Erde.
All of these are orchestral pieces (sometimes with voices), so let's stick with works for orchestra and do chamber music, solo music (largely for piano) and opera in separate posts. Another piece by Stravinsky that fits is the Symphony of Psalms from 1930, one of those brilliant commissions by the Russian conductor Serge Koussevitzky for the Boston Symphony. Another one was the Turangalîla-Symphonie by Messiaen, completed in 1948, also for the Boston Symphony.
Great German orchestral works might include the Violin Concerto by Arnold Schoenberg and the Violin Concerto by Alban Berg. A lot of their most important works are chamber or vocal music, so not included here.
The 20th century saw the growing importance of composers from outside the core nations of Italy, France and Germany, who had dominated music for so many centuries. Foremost among these are Bela Bartók whose Music for Strings, Percussion and Celesta (1936) is one of the loveliest in the first half of the century. Another great piece from that decade is Shostakovich's Symphony No. 5, written in 1937. While we are visiting Russia, we could include Prokofiev's Classical Symphony (1917) and Piano Concerto No. 2 (1923).
While we are talking about piano concertos, there were some great ones written in the 20th century such as the three by Bartók (all good, but let's pick the Piano Concerto No. 3 from 1945).
Also part of the 20th century canon would be some music by Charles Ives. I would pick the Three Places in New England (Orchestral Set No. 1) from 1914 rather than one of his symphonies.
You might have noticed that all these works are from the first half of the century. I will do the second half in another post. That will be the really difficult one because there is so much to sift through and the really important pieces have probably not all surfaced yet. It really takes about a hundred years for what is essential to become clear. Semi-clear, at least!
Ok that's a rough try at a list of the core canonic pieces of 20th century orchestral music up to 1950. Now, as is traditional on the internet, you can all weigh in in the comments and tell me about all the ones I missed. Of course, some of those were intentional.
Let me muse for a moment on how I approached this. It was largely intuitive: I did not pore through long lists of 20th century works and then hone it down. Instead I just thought about what orchestral pieces from the first half of the century really stick in your mind--which ones are really essential. I'm sure I missed some important ones, so here is your chance to remind me.
Just picking a couple from the list for our envoi today, here are first, Bartók's Music for Strings, Percussion and Celesta. This is from the Deutsche Grammophon box of Boulez conducting Bartók:
And next, Charles Ives' Three Places in New England. This is the BBC National Orchestra of Wales conducted by Nicholas Collon:
UPDATE: A frequent commentator helps me out by mentioning Sibelius. I'm pretty sure I was thinking of him but got distracted somehow. But yes, some of the finest orchestral works of the first half of the 20th century are the symphonies of Jean Sibelius. The Symphony No. 2 was written in 1902, so everything from then on qualifies and I would include all of them. I have never been as fond of the tone poems like Finlandia and Tapiola, but that might just be me. This reminds me of another omission: the tone poems of Richard Strauss of which we might want to include Also sprach Zarathustra.