I have a dirty little secret. I'm the only person in the world who thinks Tchaikovsky's greatest symphony is his first.
The 'Big Three', symphonies, Four through Six, are of course magnificent, even though they've been overplayed for literally a hundred years. If you've gone to more than half-a-dozen orchestral concerts in your life, I guarantee you've heard one of the three. Great as those three works are, there's something about them that's too unsubtle, too imbalanced, too bombastic. The right to express things so frank should feel more earned than Tchaikovsky makes it.
Some creators like Mozart and Beethoven, or Shakespeare and Tolstoy, have so many facets that it's as though the entire world speaks through them. There aren't too many people who would say that Tchaikovsky's genius, incontrovertible as it is, is quite on that infinite level. Usually, the compulsion to create art (or anything else really...) comes from creators who never quite feel whole. The paradox of this problem is that the greatest art usually comes from creators who manage to complete their sense of selves in their art. Nevertheless, at his best, Tchaikovsky gets to the level of his idols: Mendelssohn, Schumann, and especially Mozart... Perhaps it's odd to think that a composer who so prized the balance of music like Mozart and Mendelssohn so utterly unlike theirs. Nevertheless, the few moments when Tchaikovsky reconciles those two sides of himself: like in the three Ballets (Swan Lake, Sleeping Beauty, and The Nutcracker), certain of his operas which are still too seldom performed, and the First Symphony, he writes things that would make Mozart himself proud to call his own.
Russia is the land of the novel, so let's call these two sides of Tchaikovsky's personality 'Turgenev' and 'Dostoevsky.' The 'Turgenev' side is the Europhile, the lover of pretty sounds and beautiful melodies, the aristocratic Russian conservative deeply entrenched and comfortable with the establishment. Perhaps it goes a little too far into triviality, but it's thoroughly enjoyable. You can hear this Tchaikovsky in his chamber music, the piano music, the orchestral suites and serenades. But then there is the 'Dostoevsky' side to Tchaikovsky: the wounded animal, tortured soul who must confess his sins; the creature of excess, pathologically attracted to danger, the Russian who finds so much German music wooden and inexpressive. This is the Tchaikovsky that could never reconcile his homosexuality with the demands of the society in which he moved, and was clearly tortured by it. Even in our day, Vladimir Putin's Culture Minister denies the overwhelming evidence that this national icon was gay.
Tchaikovsky's First Symphony, subtitled 'Winter Dreams', is the only time in the symphonic sphere that Tchaikovsky truly reined in his inner demons. Even in the Second, the Slavophile runs rampant everywhere even if the manic depressive doesn't, and whereas there's a beautiful melody everywhere you turn in the First Symphony, there's only one beautiful melody at the beginning of the Second. In 4, 5, and 6, everything turns into excess. The beautiful melodies abound, so does the tension, but rather than trying to delight or move the listeners, Tchaikovsky overwhelms them.
This is the first truly Russian symphony. Whereas Brahms and Beethoven create a symphony by taking a few notes and put it through every conceivable permutation, Tchaikovsky creates his by working fully-fledged melodies into his structures. Some find this conception of the symphony to trivialize it, but approachability is not a sin, and it created a parallel symphonic tradition in Russia that distinguished it from the German model. As soon as the symphony begins, you feel what must be the chill of the Russian landscape and the snow that glistens on the windowsill. Your mind can't help but form similar wintry pictures in the other movements, where the vast Russian expanse is felt with all its snow and permafrost and cold air. Within a minute, you hear both the beautiful melodies and manic desire to explode from them held in perfect balance. A balance sustained magnificently through the whole of the symphony. You could never guess that Tchaikovsky nearly had a nervous breakdown while writing it. It's nothing less than a Russian Beethoven's Fifth.