I always like threads like these; there's so much good music out there, and some of it gets overlooked sometimes.
I'll mention Martinu again. I've actually been on a Martinu kick lately, finally picking up Vaclav Neumann's superb cycle of all of Martinu's symphonies. They are all at least very good. The string quartets, piano concertos, cello sonatas, the great The Epic of Gilgamesh, and the opera Julietta all deserve mention, I think. Martinu is still too underappreciated I think.
Erwin Schulhoff is an almost unknown composer, and that's a shame. His chamber music, in particular, is superb, his string quartets 1 & 2 being among my favorites from the 20th Century, which is saying a lot. His Hot Sonata for piano and saxophone sounds just like 20s-era jazz, but it's a composed piece. His Concerto for Piano and Small Orchestra (a misnomer if ever there was one!) is splendid. His symphonies are good, but not up there with the best, and his main stage works, the ballet Ogelala and the opera Flammen are both very good.
A bigger name, but not big enough, is Gabriel Faure. His great Requiem is of course well known, and deservedly so, but his solo piano music rates with the finest in the canon, and his violin sonatas are sublime.
And I’ve mentioned him before, and I’ll no doubt mention him again and again, but Karol Szymanowski is criminally neglected. His 3rd and 4th symphonies are masterpieces, his big piano pieces (the 2nd and 3rd sonatas, the Masques and Metopes) as well. And then there are his string quartets, his staggeringly great Stabat Mater and his magnum opus, Krol Roger. Man, he is simply amazing.
Alexander Zemlinsky is another woefully underappreciated composer. His string quartets, the last two of the four in particular, and his Lyric Symphony, choral works, orchestral works, and, above all, his operas – led by Der Konig Kandaules and Der Traumgorge, at least for me – are all worth serious consideration by anyone.
One last name for now: Ahmed Adnan Saygun. I’ve only heard his piano concertos and string quartets, but those are all quite worthy works. Bartok is an obvious (and heavy) influence, but surely I need to hear his symphonies . . .