Eric343,
Don't have a favorite, but some must haves that have not let me down:
Mahler Symphony No. 2 in C Minor "Resurrection" Leonard Slatkin and the Saint Louis Symphony Orchestra w/ Kathleen Battle and Maureen Forrester on Telarc.
Big, grand orchestral work using the entire orchestra. The second disc is the vocal portion.
Copland disc featuring Fanfare for the Common Man Zubin Mehta and the Los Angeles Philharmonic Orchestra on London.
Various Copland pieces, but Fanfare will test any system's lows (timpani), mids and highs (brass). Appalachian Spring is a very beautiful, peaceful piece, lots of woodwinds especially the oboe.
For the ambient(?) in me, anything by Portishead and Massive Attack especially Angel on the Mezzanine album. Listen for the added static on various songs to make it sound like an LP.
Regards - reynman