Let's start with France as I find it helps to look at specific traditions in sorting out who's who. Olivier Messiaen is an obvious choice. He made the pre-1950 list with his Turangalîla-Symphonie, one of the most striking pieces of orchestral music of the era, but there are some good candidates from after 1950 as well. Messiaen lived until 1992 and was very productive. I think I would pick out two pieces, Et exspecto resurrectionem mortuorum (1964) for wind, brass and percussion and Des Canyons aux étoiles for piano, horn, glockenspiel, xylorimba and small orchestra (1974). Another composer very active in the second half of the century was Henri Dutilleux, a lapidary composer who wrote a select few outstanding pieces. Among them, perhaps the most memorable are two concertos: one for cello, written for Mstislav Rostropovich titled Tout un monde lointain (1970) and one for violin, written for Isaac Stern, titled L'arbre des songes (1985).
German composition post-war was very different from before simply because of the desolation of war and the departure or loss of Jewish musicians. The sense was that everything had to begin anew, a blank slate as it were. Composers like Karlheinz Stockhausen found themselves in the class of Messiaen at Darmstadt. In the 1950s and into the 60s, there grew up a "Darmstadt School" of composition that included, apart from Stockhausen, Pierre Boulez, Bruno Madera, Luigi Nono, Luciano Berio, Mauricio Kagel and others. Characteristically, their music was avant-garde, experimental, and, at least in my opinion, hasn't aged well. I think that if we regard "canonic" works as ones that have a special aesthetic appeal to audiences, then hardly any of this music has become part of the canon. That phrase "hardly any" is rather weaselly! So let's have a look: are there any possible candidates? Two that come to mind are Gesange der Jünglinge by Stockhausen, an early electronic piece, and the Sinfonia by Berio. I think that my readers should weigh in on this. Do you think they have achieved canonicity?
Let me know in the comments.
More and more, as the century progressed, we see composers from outside central Europe, not just Russia and North America, but Poland, Greece, Great Britain and Japan. Shostakovich wrote some wonderful symphonies post-war of which I think that the Symphony No. 10 (1953, but possibly completed earlier) is perhaps the most-enjoyed (that is the basic measure of canonicity). There is one enormously popular symphony by the Polish composer Henryk Górecki, the Symphony No. 3 the "Symphony of Sorrowful Songs" of 1976 that even crossed over to the pop charts in Europe. British composers became more and more prominent after the war, particularly Benjamin Britten, though his strengths are perhaps greatest in opera and vocal music. Still, his Young Person's Guide to the Orchestra and the Sea Interludes, adapted from the opera Peter Grimes, would likely qualify. Unfortunately, due to my oversight, they should have been included in the first part as they were composed before 1950! Sorry, Britten fans.
After all the experimentation in the 50s and 60s, a new kind of approach arose in the 1970s with the so-called "minimal" composers whose music, though featuring a steady pulse and lots of repetition, was hardly minimal. The two names to note are Philip Glass and Steve Reich and I think that there are likely canonical works from both of them. But we have to bend the category a bit: they both tend to write music for unusual ensembles that are somewhere between what we usually think of as chamber music and orchestral music. An example, and I think a piece that is sure to be in the canon, is Steve Reich's Music for Eighteen Musicians:
Philip Glass, on the other hand, has written quite a bit for conventional orchestra and the Symphony No. 3 from 1995 is a good candidate for the canon:
Going even further afield, the first composer from Japan to have real recognition in the West was Toru Takemitsu and he might have a bid for canonicity with the piece November Steps for biwa, shakuhachi and orchestra from 1967:
From here on, the number of composers multiplies enormously and the difficulty of sorting out the exceptional pieces becomes harder and harder. For one thing, there are so very many works--and even composers--that I simply have not heard. Here is where my astute and knowledgeable commentators will undoubtedly make a contribution. What have you heard that you think will make the cut?