I would venture to say that Stravinsky's use of the changing rhythmic patterns constantly is very progressive like "Dumbarton Oaks" concerto. Probably the moodies is a good start for prog rock. Who knows, this might be like discussing politics or religion it is the never ending story and it depends upon taste.
ELP and Pink Floyd were my first contacts with prog rock. And I've been a fervent fan ever since.
It certainly marked my life as a musician and composer since my bands have always been progressive at heart.
