Final answer:
Arnold Schoenberg is often regarded as the leading avant-garde composer of the 20th century, especially for his developments in atonal music and the twelve-tone scale. His work, along with Igor Stravinsky's 'The Rite of Spring', pushed the boundaries of traditional Western music.
Step-by-step explanation:
When discussing the leading avant-garde composer of the 20th century, Arnold Schoenberg comes to the forefront. He was instrumental in developing atonal music and had a profound impact with his creation of the twelve-tone scale. These innovations truly challenged and expanded the boundaries of Western music, contrasting with what listeners were accustomed to since the Renaissance. Schoenberg, through his compositions, subverted traditional expectations by incorporating dissonance and unexpected rhythmic patterns.
Another seminal figure of the period is Igor Stravinsky, whose groundbreaking work 'The Rite of Spring' caused quite a stir for its avant-garde approach and has since become one of the most influential pieces of modern classical music. While others like Berlioz, Strauss, and Wagner also contributed significantly to the era, it was Schoenberg's radical defiance of Western musical tradition that marked him as one of the most innovative and influential avant-garde composers.