If we're talking influence, I would nominate:
Machaut: Messe de Notre Dame - first major use of polyphony
Monteverdi: Orfeo - I believe the first major work which used harmony,
rather than modality
Schoenberg: String Quartet No. 2 - the work which rid music of a hierarchy
of tones
The influence of all three are obvious, but one may very well ask, with
George Bernard Shaw, whether being first in art isn't a lesser thing than
being last. In other words, is your importance determined by who came
before you or who came after? Bach was not the first, nor was Mozart.
Steve Schwartz