Not technically a Symphony as such (since it's title is "Concerto"!) but certainly on a symphonic scale is Bartok's famous Concerto for Orchestra.
For me it succeeds on so many levels, not the least of which is its primary purpose, ie, to showcase an orchestra which it does in spades. Even a cursory analysis reveals that some of the harmonies are surprisingly conservative, it's just the rapid manner in which Bartok passes through them that makes it sound "modern." His clever evocation of Hungarian-style melodies is also a winner. Also marvellous is his use of form and his overall compositional technique (also surprisingly economic - but it's what he does with his small ideas and how he does it that make it work on the grand scale it has become).
Aside from the technical/analytical things, it's really quite an exciting listen. I feel like I've been on a long journey through exotic places after listening to it from start to finish.
Similarly - but with a VASTLY different landscape - is Messiaen's Turangalila Symphony, although I will concede that the listener probably needs to be in a Messiaen mood for that one!
