Agreed…I enjoy both Riley and Penderecki’s compositions Unfortunately Mr. Penderecki passed away a few years ago.
I guess if I had a 20th century list I would add Penderecki & Witold Lutoslawski…my list would be much longer…so many outstanding composers in the 20th century!
On the balls front: Mason Bates, Thom Willems, Paul Hindemith, Igor Stravinsky, Aaron Copland not yet mentioned.
Opera: Karol Szymanowski, Leos Janacek and Puccini didn’t die until 1924, leaving Turandot unfinished, marking the end of Italian opera.
The Viennese school, Mahler, Schoenberg, Berg, etc,