Being the modernist prick i am, Sibelius 7 is still perhaps THE orchestral work for me, the one that always gets my full attention, from which there are new aspects to be found every time, and the one that gets a tear in my eye.
Actually, i think 7 was very bold in its own way -- i love the thematic structure, with each orchestral group having their own theme. (Well, sort of.) It definitely isn't a regressive piece, that's for sure.