Symphonists Supreme

This makes a cheerful story ~ but apparently, in the light of forensic scholarship, that's about all it still is!

This piece is now the European Anthem

This is (usually) as much of a 'recipe' as the theatrical convention of a five-act tragedy, or indeed a traditional three-course meal

Even within the canon of his own music, let alone Classical Music as a whole, this piece is rightly regarded as sublime

This was Tchaikovsky in his 6th ('Pathetique') Symphony. The movement, though very different in atmosphere, probably shares with Mars (from Holst's Planets Suite) the distinction of being one of the only two widely-recognised pieces in this time-signature for the modern concert orchestra

This was Berlioz' extraordinary work of symphonic autobiography-cum-fantasy, complete with a Witches' Sabbath round-dance and a March to the Scaffold in tumbrils. No other work in the standard repertoire could be quite like it!

Plenty of other composers produced different totals (Haydn: 104, Mozart: 41, etc), but there does seem to be something quite coincidentally special about this number. Musicological scholarship (bless it) does its diligent best to disprove and diffuse such notions, but they linger in the popular imagination! (Schubert's last symphony was, of course, memorably and poignantly 'Unfinished'.)

Each of the other Answers refers to an intentional musical or para-musical effect by the composer: a ticking clock, the sudden loud chord to awaken slumberers a few moments into a quiet slow movement, the players' pantomime about wanting to go home. The 'Miracle' Symphony was otherwise relatively ordinary, except that a massive chandelier collapsed (Phantom-of-the-Opera-style) onto where the audience had just been seated ~ just after many of them had surged forward to applaud the performance, and were safely out of its way. We can be fairly sure this was not an example of Haydn's humour or forethought!

The subtitle was never sanctioned by the composer, but a huge orchestra is called for (e.g. 5 flutes rather than the usual two or three), plus substantial numbers of singers, and it would be a simple enough gesture to summon a choir several hundred strong ~ if you were already onto such a whopping logistical scale. Many years ago the BBC made an interesting radio documentary entitled 'Mahler needs a jumbo', about such challenges. Berlioz (Answer 3) meanwhile had written a Grande Messe des Morts ('Great Mass of/for the Dead') calling not only for beefed-up versions of the classic oratorio forces, but also no fewer than four offstage brass ensembles

There are pieces entitled Symphony (or similar: the term means 'voices [or parts] together'), by Handel ~ for example, the short instrumental representing the shepherds in Messiah and others; but the substantial orchestral suite of three or four movements for 'classic' orchestra was created by Haydn, who 'tamed' the outdoor hunting-horns of his aristocratic patron and brought them into the orchestral fold for the purpose