Matter of opinion, and perspective helps--we can be more certain about who broke new ground if it happened long ago, so that we can see the effects on subsequent compositions. When we get to post-1950, it's safe to say we're guessing.
And it might help you as a composer more to read some intriguing books about the theory of composition, say, Cage or Hindemith. Or books about musicianship in general, even going back to the old Baroque treatise on flute-playing by Quantz.
Well, let's see, back to your main question. From music history, the following were "groundbreakers"--
Medieval: Leonin and Perotin
Renaissance: des Prez, Dufay
Baroque: Claudio Monteverdi
Classical: Boyce, Haydn
Romantic: (oh boy) Beethoven (Pastorale and Ninth), Hector Berlioz, Chopin, Scriabin, Wagner, Mahler, and I'm pretty sure I'm forgetting someone...
Modern: Eric Satie, Debussy, Stravinsky, the atonalists (Berg Shoenberg Webern), Kodaly, Orff, Ives, Copland (arguably), Louis Armstrong and his ilk, Benny Goodman and their ilk, Bill Monroe, and the ragtime-era tunesmiths.
Postmodern: John Cage. Now, if you want to look at pop and jazz, there are dozens, including Miles Davis, John Coltrane, Sun Ra, Mahavishnu John McLaughlin, Bobby McFerrin, Dave Brubeck, Rahsaan Roland Kirk, the Blues Project, Joni Mitchell, the Moody Blues, Ani di Franco, Dave Matthews, that whole generation of lead guitarists who made it a standard voice in rock including Hendrix-Clapton-Jeff Beck-Randy California-Jerry Garcia-Jorma Kaukonen-Gary Duncan...and also producers like Barry Gordy and Phil Spector.
There--hope that keeps you pleasantly occupied.
· 1 decade ago