It's highly possible that the most underrated composer in the history of music is still Joseph Haydn. There are literally thousands of underrated composers out there and probably only a dozen or a dozen and a half who are genuinely overrated, but at least in my opinion, the founder of modern music is Haydn, not Bach.
I've written at least three or four long blogpost essays nobody reads about how Bach's place in music is overvalued, and I really don't want to write a fifth, but there is something about Bach that, to me, is too heavenly, too divorced from the human experience, too, as we Jews say, 'goyish.' When you get to know Mozart and Beethoven's music, they take on the quality of old friends, but when you get to know Bach, both the glory and the weakness of it is that it never stops being an extraordinary experience. The music is too perfect. There's expression aplenty in Bach, but it's not human expression, it's the expression of divine beings. These divine beings might feel compassion for our suffering, but they can't relate to it, and they rarely ever seem to relieve it. Furthermore, where's the humor in Bach? And most importantly, where's the fallibility? You can search far and wide for a compositional weakness in Bach, and you might find one every million measures. That, as far as I'm concerned, is the ultimate compositional weakness. It's like music assembled by a kind of celestial computer coding. He was so masterful at counterpoint that through his counterpoint he practically created the common harmonic language we use to this day.
But this was entirely an accident. Very few composers knew more than a few pieces by Bach until Mendelssohn worked mightily to revive him in the 1830s to a fame Bach never had in his lifetime, and performed Bach with a seraphic beauty which was antithetical to instruments and performing style of Bach's period. The Bach of the Romantic era was a completely different composer, grounded in harmony rather than counterpoint. As a personal interjection, when I hear Bach performed on original instruments, rendered little different from a generically excellent composer of the Baroque period, I begin to understand why Bach's employers in Leipzig offered Bach's job to Telemann or Graupner.
Bach's music is pure counterpoint and harmony, there is no flash, there is not even really a style of which one can speak. There is only pure substance; as the musicologist Jan Swafford put it, nobody ever wrote better notes than Bach. In the same way that Immanuel Kant is often considered pure analysis and thought and system that gives little consideration to stylistic clarity or common wisdom for layman readers, the vast, vast majority of Bach's is a pure contrapuntal and harmonic thought that gives little thought to instrumental color or rhythm. Both were revolutions for the humanities that gave them something like the rigor of science and for this pseudo-intellectual, both revolutions achieved more rewarding results when their successors turned their soft science back into art.
Whether it was actually Bach who actually codified common practice tonality or a combination of Bach, Handel, Vivaldi, Scarlatti, Rameau, Telemann, Pachebel, Froberger, Graupner, Kuhnau, and who knows how many other composers, what is much less deniable is that it's Haydn who first utilized how to use common practice tonality to the best effect. His symphonies, his string quartets, his piano trios, are little dramas of harmony. Platonic dialogues carried out in sound between two musical characters who propose ideas that seem to contradict each other in every way, but they work through every conceivable implication of their ideas until they can arrive at some kind of resolution, some synthesis, some livable compromise that allows life to go on.
The Symphony, Haydn's great contribution to the literature of the world, and arguably the greatest of all artistic contributions because it's literally the only place in the world where the abstractness of philosophy has demonstrable physical presence, is about the continuity of life, the pacification of the spirit. Perhaps I'm too much a fundamentalist in this regard, but having just taught a course on symphonies, I'm more in awe of them than ever before. I believe that in a way that songs and operas and ballets never could, absolute music gives us a way of coping with life's vicissitudes. If Greek tragedy is a ritual that purges us of our emotional tensions and repressions, then the grand symphonic tradition gives voice to those emotional tensions and repressions which are too primal for us to attach words. And when we hear them, we understand that there are dimensions out there where our anguish and contraditions are understood, and a place, somewhere in the universe, where they can be resolved. And knowing, as we now do, that there is a chance for some kind of resolution in some far off place, whether it's heaven, or somewhere in the elements of the universe, or simply in the dimension of music, we're free to look at the dimensions of life with much more equanimity, and solve our personal problems without the weight of unresolved passions bearing down on us. The problems of this world were never meant to be transcended, but thanks to music, we can direct the human urge to transcendence to the only dimension we know of where transcendence is truly possible, and the place where it's most possible is the symphony. And the transcendent possibility of the symphony begins with Haydn.
Beginning of Haydn 44 "Trauer" 2nd movement (Fricsay/RIAS)