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 wonder if I understand why Bach's employers in Leipzig offered Bach's job to Telemann or Graupner before him.
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 philosophy and abstract thought 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)
And yet, in order to get to transcendence in the manner which we get in so many later symphonies, were the symphony becomes like a battleground of diminished and minor chords against which major-key happiness and triumph has to emerge transcendent, we first need the Haydn formula in which music is a conversation between light and darkness. Like in every high comedy, The Simpsons for example, every dark sentiment has to be immediately balanced so that the darkness isn't taken too seriously. In such a world where the imagination can't allow itself to get too turbulent, it's inevitable that formula creeps in. Just think of the Simpsons after Season 8 or 9...
In Haydn's case, his later Simpsons episodes came earlier in his career, when he was still figuring out how to make symphonies into immortal masterworks. Some of them have innovations that are truly astonishing, but with a number of exceptions, don't assume that early Haydn is as interesting as lots of musicologists want us to believe. There's no reason to listen to all 104-and-change Haydn symphonies. Start by listening to the last twelve, the London Symphonies, 93-104, when Haydn polished his equanimity of expression and wit to an agility that seems to float like a butterfly, sting like a bee. Then, perhaps, listen to his Paris Symphonies, 82-87, which are meant as grander statements for a larger orchestra, proto-Beethovenian perhaps though Haydn could have no way of knowing - and yet chamber orchestras always play them. Fill in the gap with the five symphonies in between 88-92, fulfilled for other commissions, and many people feel #88 is the greatest of all - though for those who care I'd probably put it in the top 5 with 92, 98, 100 and 103. Then listen to all the 'Name' symphonies. 29 of Haydn's symphonies have names. So far as I know, Haydn never gave any of his symphonies a name, but if a Haydn symphony has a name, that means that the symphony made an impression on someone over the last 200 years, and thought enough of it to call it something unique. Past there, there are a few worth getting to know, but as a homework assignment, I think that's more than enough.
Haydn, having capped his symphonic achievement with Symphony #104, a perfect fusion of light and darkness, has nowhere else to take the symphony after 1795. Anything after Haydn 104 would be a formula. The Symphony, taking place both in a world of metaphysical abstraction and also pitched to an aristocratic audience of this world, cannot at this time be heard to suffer unless the suffering is balanced evenly by joy. But the world itself was suffering - in the Reign of Terror after the French Revolution roughly 40,000 died with 20,000 sent to the guillotine, followed by the Vendee Rebellion, which may have killed 700,000 Frenchmen, along with all the various revolutions that make up the total French Civil Wars from 1789 to 1801, in which a probable total of 1.4 million died. We then follow this with nineteen years of Napoleonic War, the highest estimate being that six million died in those. Haydn's symphonies had no way of coming to terms with a world in which such bloodshed was so visible, but his choral music.... (Nelson Mass Opening First 50 seconds/Peterson/Oslo Camerata)
Sacred music, particularly the Mass, is pitched to God and Christ and the Holy Ghost and the Virgin Mother, and are precisely about the transfiguration of suffering. What we just heard was the beginning of what we now call the Lord Nelson Mass, named for Britain's heroic Admiral who lost his arm and then his life in the Napoleonic Wars. The Latin title is "Missa in Angustiis", or Mass for Anguished Times. It was written in 1798, and can be seen as a sequel to Haydn's mass of the year before, the Mass in Time of War which begins in serenity and ends in terrible turmoil. The Mass for Anguished Times is consistently dark in a spirit that perhaps no other of Haydn's voluminous works shares. The only real precedent for it is Mozart's unfinished Requiem and the hellish scenes of Don Giovanni, and at this point in history, Beethoven is still known far better as a pianist. 1798 is seven years after Mozart's death, five years before Beethoven's Eroica. Austria, perhaps the world, had only one verifiable musical genius to articulate the spirit of the times, and the spirit of the times were only getting darker.
The elderly Haydn was a seasoned traveller who'd spent much time feted in France and England, but he was forever a loyal subject to the Austrian Crown and the Holy Roman Empire without a subversive bone in his body. He was descended from Flemings and Croats, but the fate of Austria, it's triumphs and tragedies, were his. The Austria of his generation, ruled by Empress Maria Theresa, deeply Catholic, conservative, and declining, depleted first by the War of Austrian Succession in the 1740's, then by Prussia's conquering of Silesia, the most profitable Austrian province, in 1756, meant that the Austria of the time, like so many declining powers, turn especially to culture because they have to find their pride in something other than political achievement. Thanks to Haydn, they found it in music. And while France, until the 1780s at least, had a really great 18th century politically and especially intellectually, they found that their very achievements were exactly what pushed them into a generation long apocalypse.
Austria, on the other hand, had already known real suffering and decline and deprivation. Austrians were not taken in by utopian delusions of the world being anything but what it is. While France was in thrall to Jean-Jacques Rousseau, with his notion that man is born free yet everywhere he is in chains; the German speaking lands were in thrall to the still very young Johann Wolfgang von Goethe, who wrote that what little freedom remains to humans so fills them with fear that they seek out any and every means to be rid of it.
Napoleon would not yet be First Consul of the French Republic for another year, and not Emperor of the French for another six. But the decisive moment in the fortunes of the French republic came in 1796 with the appointment of this semi-Italian semi-soldier of fortune, not yet thirty years old, to be Commander in Chief of the French armies in the Wars of the First Coalition - pursued by a collection of European monarchies lead by Britain and Austria, terrified of the French Revolution spreading to their Empires as they would be for another hundred twenty years until even the upper classes were compelled by World War I to admit that sovereign monarchy is not a viable option in the chaos of the modern world.