Wagner

He did not want opera but a total work of art, with music, poetry, staging and light controlled by one person. To get it he built his own theatre — the Festspielhaus at Bayreuth, with the pit hidden beneath the stage so the audience cannot see the orchestra, a radical redesign in 1876.

The four operas of the Ring took twenty-six years. His harmony prised open a gap in the very first chord of Tristan und Isolde; that unresolved Tristan chord is usually taken as the point where tonality began to come apart. The music has to be heard separately from the man: he wrote vicious antisemitic tracts, they were taken up by the Nazis after his death, and that is not a fact anyone can route around.