1. John Cage (1912-1992): Cage was an American composer, music theorist, and artist. He is widely regarded as one of the most influential composers of the 20th century. Cage's work was often experimental and avant-garde, and he is best known for his use of chance operations in music, as well as his conceptual works, such as "4'33"" (1952), a piece of music consisting of four minutes and thirty-three seconds of silence.
2. Pierre Boulez (1925-2016): Boulez was a French composer, conductor, and music theorist. He was a leading figure in the development of serialism and electronic music, and his work was often characterized by its complex and dissonant nature. Boulez was also a prolific conductor, and he led many of the world's leading orchestras.
3. Karlheinz Stockhausen (1928-2007): Stockhausen was a German composer, music theorist, and teacher. He is considered to be one of the most important figures in electronic music, and his work often explored the use of spatialization and sound synthesis. Stockhausen's music was often complex and challenging, and he was not afraid to push the boundaries of what was considered to be music.
4. György Ligeti (1923-2006): Ligeti was a Hungarian-Austrian composer, music theorist, and teacher. He is best known for his use of microtonality and polyrhythms, as well as his exploration of new sonorities and textures. Ligeti's music was often complex and dissonant, but it was also often beautiful and lyrical.
5. Steve Reich (b. 1936): Reich is an American composer, music theorist, and teacher. He is a leading figure in the development of minimalism, a musical style characterized by its use of simple, repetitive patterns. Reich's music was often trance-inducing and hypnotic, and it explored the relationship between time, space, and perception.
These are just a few of the many leading composers of experimental music in the 1900s. Their work has had a profound impact on the development of contemporary music, and it continues to be studied and performed today.