Wozzeck
Wozzeck was the first major opera written in a modern 20th century style.  A lot of the music is atonal (not in any key) so Berg was able to use this to express the madness of Wozzeck.  Sometimes the music becomes more tonal, especially when the story is telling about love and humanity.  He also bases some of the music about the soldiers on folksong which he treats in his own special way.