In the last 20 years I've found around 340 20th century composers, all of
them with very interesting works.  But talking about a brief random list,
the first names that come to my mind are:

Webern, Schoenberg, Alwyn, Stravinsky, Tubin, Messiaen, Kodaly, Britten,
Schulhoff, Ciurlionis, Caplet, Bloch, Henze, Honegger, Weill, Martinu,
Ravel, Janacek, Korngold, Panufnik, Bartok, Eisler, Lazarof, Dutillieux,
Lyatoshynsky, Sallinen, Vaughan Williams, Rozsa, Poulenc, Holmboe, Milhaud,
Walton, Szymanowski, Debussy and . . . many more.

Dave.
Guadalajara.