Other other-than-Schubert ones worth a lookhear may include Franz Johannes Gleißner (1761-1818 so more Mozart's generation actually), Friedrich Witt (1770-1836) (who I see JimL has mentioned), also Krommer, Czerny, Reicha, the Vranicky brothers of a recent thread, ... and some of those other usual suspects of music that we know in other spheres (Reicha and Danzi and Krommer from their wind, not their symphonic, music, e.g.) - and Vorisek e.g.?
Google Books (
here for example is a list of interest to be found in that book) provides some preview pages of "The Symphony in Beethoven's Vienna" that provide some interesting context, some of these names and some information on their works. Including a symphony by Czerny that isn't one of the 6 that I think appear in Groves?- (in D major but not no.2 - Scherzo in the wrong place and mode (D minor), indeed none of the movement descriptions are right. Written, I think, around 1818 and earlier than his op.780 symphony no.1 in fact.) Greatest, though? For that question? (And for that question
I'm not quite convinced it's Schubert... - I don't dislike his music but that doesn't mean my answer to that question becomes yes) - you've got me, though I'll give it some thinking. (In quartets, in the last few, with Schubert- there I can see it. Not last-few-Beethoven-quartet level- practically nothing is, in my book - but very, very high.)