I would approach this in a slightly different way, given the underlying concern of the original question: from the 200 years prior to 1960, how many symphonies deserve to rank among the greatest? That is, how many symphonies utilize the full resources available to the composer in the fullest manner? How many are both structurally complex and powerfully emotional, operating consistently at the highest level?
Not so many as we might suppose, I would guess. A certain number by Haydn, but no more than 20 or 30. The last three by Mozart, and perhaps a couple of others. Beethoven 3, 5, 6, 7, and 9 (and some might argue against 6!). Schubert 8 and 9. And then....? Remember, we're talking 'greatest', not 'very good' or 'most popular'.
So (he says, rushing in where angels fear to tread)-- Nothing before Brahms and Bruckner, or at most Berlioz Symphonie Fantastique, Mendelssohn 4, and Schumann 4. Brahms 1 (though this would not be universally accepted, judging from various reactions over the years), and certainly 4 (2 & 3? |Do they really hold up against the aforementioned pieces by Beethoven?). Bruckner 4, 5 and 7, possibly, and certainly 8 & 9. Saint-Saens 3? Tchaikovsky 4, maybe 6. Dvorak 7 probably, and I'd make a case for 8, as others would for 9. Mahler 6, possibly 9. Nielsen 4 &5. Sibelius 4 & 5? Shostakovitch 5, maybe 8, and 10. Prokofiev 5? A couple by Miaskovsky? A couple by Hartmann?
And then, since I'm no doubt forgetting a few (Toch? Martinu? Tubin?), we'll say that another dozen can be added. This gives us, at most (assuming truly great symphonies don't come pouring out of the woodwork), some ninety or so truly great symphonies in two hundred years (simply accepting every symphony mentioned above as 'great'). Take off Haydn and Mozart, who wrote in a vastly different era and other vastly different circumstances than their successors, and the number drops by almost half. So we have an average of somewhere between fifteen and forty-five great symphonies for each fifty years. Just on the numbers I'd suggest that the last fifty years are holding up pretty well, even if you double the above numbers.
My nominations (sticking, almost, to the original three symphony limit)?
Shostakovitch 13 absolutely, and 14 and 15 strongly.
Pettersson 7, and maybe 8.
Aho 7
There are, though, many other very fine symphonies of recent origin. As I said, the last fifty years seem to be at least as productive as any other fifty year period after Beethoven.