Though not symphonic, I quite like 'Spem in Alium' by Thomas Tallis.
As for actual symphonic pieces, I'm not a huge fan of old stuff though (doesn't mean I don't like it, means there's stuff I prefer over it) there are a few treasures in there. I've actually been into more modern stuff - movie and game scores - mainly for interesting uses of instrumentation (or translations from their 8-bit originals) and how the music is often written to tell a story or highlight a theme in the game/movie.
Some that I can think of off the top of my head:
Clive Mansell - Lux Aeterna
Nobuo Uematsu - Terra's Theme (orchestral version) <- or pretty much any other orchestration from old MIDIs used in the FF series
Various bits and bobs from Hans Zimmer - The Rock, Crimson Tide
Mozart - Requiem
Pergolesi - Stabat Mater
There's probably heaps of others I haven't even though of - but yeah, I do like to my listening balanced with other stuff from my usual fare