Yeah, for the sake of simplicity I narrowed it down to post-Beethoven but still more or less common practice (although Wagner begins to move away from common practice and even the modal inflections of certain romantic composers does as well).Romanticism does not have an abrupt end... in fact, it continues and leaves us to try to distinguish the end of that period, when it really merges into 20th Century eventually. Except in some extreme cases (Xenakis, Stockhausen), I cannot clearly trace any perfect division here. Of course, this is only important if your favourite Romantic symphony belongs to this transition period.
MoonlightSonata, you have to realize that PetrB's various opinions on Beethoven are, um... well, he isn't the only opinion! There are ample resources (musicologists, musicians, and the like, which he seems to value so highly, and rightly so) that would say otherwise. Books, essays, and articles have been written on this subject. I don't think simply declaring it "not a romantic symphony" has turned the tide. Lol. It was in his Late Period in which Beethoven often broke free of the "Classical Idiom". From the "American Bach Soloists" article on the Ninth, "Beethoven's Ninth Symphony has always been a masterwork that simultaneously creates a chasm-through the nature of its wide range of summative styles and form-and bridges its own gap. It pushed the instruments of its generation to the limit, and has, for nearly two centuries, elicited highly personalized readings and interpretations. Beethoven's Ninth Symphony is often described as a work which broke free of "Classical" style. Just as one can cite the music of Richard Strauss as having taken the Romantic style-even diatonic harmony-to its very outer limits".If Beethoven's Ninth counts (which apparently it doesn't), that would be my nomination.