BBC Music's September issue features "The 20 Greatest Symphonies of all time." What they did was get 151 conductors, many but not all well-known, to offer their choices of the 3 greatest symphonies each. So the result isn't really the 20 greatest symphonies of all time, but the 20 top candidates for the 3 greatest symphonies of all time. Can you see how there might be a difference?
The favored 20 included five by Beethoven (and you can guess which five), the Fantastique, all four Brahms, two Bruckner (Nos. 7-8), three Mahler (2, 3, 9), two Mozart (the Great G Minor and the Jupiter), Shostakovich 5, Sibelius 7, and the Pathetique. Except for the Mahlers, of course, those all get thumbs up from me, though I find Sibelius 7 difficult, while I have no trouble with his famously difficult No. 4.
The votes were included, but not the tabulation, so I compiled that myself, and found that the top 20 each received from 6 to 32 votes. There were 97 works nominated altogether, a few of them not labeled as symphonies (Bartok's Concerto for Orchestra, Mahler's Das Lied von der Erde), and 40 of them got only one vote each. The most popular composer who didn't have anything make the top 20 was Elgar, interestingly enough.
Most of the conductors chose pretty conventionally, and as the top two choices were Beethoven's 3rd and 9th, I have to conclude that the concept of "greatest" symphony melds in some minds with that of "most monumental" symphony. (That would also explain the large wad of Mahler.)
Only a few conductors chose two works by the same composer. I give special points to Alexander Lazarev, whose choices were the 3 symphonies by Rachmaninoff. (A few others also picked No. 2.) The only list of 3 that nobody else picked any of was not that eccentric: Erik Nielsen had Haydn 104, Schumann 3 (everybody else who picked a Schumann had No. 2; I'd have gone with No. 4 if any), and Schubert 9 (the only one for this masterwork? The problem with limiting to 3 is that someone like Schubert gets drowned out by Beethoven and Mozart). The most eccentric list had to be those of Kristjan Järvi, who picked Gelgotas 1, Stravinsky Symphony in 3 Movements, and Sumera 2. One other person also picked the Stravinsky, but I've never even heard of Gelgotas (who must be Gediminas Gelgotas, a young Lithuanian whose Wikipedia entry doesn't list any symphonies). Closely followed by Kirill Karabits, a fan of obscure Soviets, who picked Liatoshinsky 3, Prokofiev 5 (OK, not obscure), and Terterian 3. Honorable mention to the famously unpronounceable Mirga Gražinytė-Tyla, who picked Bruckner 6 (as did 3 others: also my favorite Bruckner), Haydn 6 (I like it too, but greatest?), and Weinberg 1 (not a work I know by this prolific and uneven composer whom I respect more than like).
What would I have picked? My first reaction would have been that I would find it impossible to pick the 3 greatest symphonies. Maybe the 50 greatest, but not the 3. On second thought I would have gone for a conventional list spread across periods: Beethoven 7 (less monumental than 3 or 9, but oh so supremely flexible), Dvorak 9 (which I believe is the closest existing work to the Platonic ideal of a symphony), and Shostakovich 10 (for which "deep" is the best adjective). For a more eccentric choice, I might go for 3 20th-century Symphonies No. 6 by big name composers but that nobody else picked: Prokofiev, Shostakovich, and Vaughan Williams, all really tough, gnarly works that I like a lot.
I wrote a few years ago about my choices for The Three Greatest 20th-Century Symphonies by Composers You've Never Heard Of: Atterberg 6, Dopper 7, and Santos 4. And, for a booby prize, here's my list of The Three Worst Symphonies by Composers Capable of Writing a Better One (And Thus Excluding Mahler): Tchaikovsky Manfred (the absolute worst symphony by a good composer of all time), Khachaturian 3 (utterly terrible Soviet kitsch), and - let's find something a bit older - Mendelssohn 5 (which is actually No. 2 in date of composition, but gets to be No. 5 because the composer had the sense not to publish it, and it only came out after his death).