Originally posted by monk
1) Arnold Schoenberg - he again and again changed his cutting edge style until he reached his 12 tone system at an age of 47.
And yet how much influence did he really have? Certainly on a generation or two of classical composers, most of whom are hardly listend to apart from in academic circles. His method was sometimes adopted by more tonal composers as a colouristic device - Britten, Stravinsky, Shostakovich come to mind - but not as central to their output. Perhaps as an influence on movie music.
Undoubtedly highly original and ground-breaking. But at the start of the 21st century hardly avant-garde as the direction of much classical music seems to have headed back towards tonality.
In fact you could put a fascinating argument together that the real 'avant-garde' of the early 20thC were the likes of Korngold who were sidelined by the serial orthodoxy of the mid-20thC but have come into their own in the last 20 years. They were the ones who prepared the way for what is actually happening today.