Gustav Mahler's Symphony No. 8, the Symphony of a Thousand.
I got to sing two performances of this as a chorister in the early 1990s with the Knoxville Symphony Orchestra. I don't even think we were that good. But it was an astonishing, unforgettable musical experience. The 1st movement is my favorite. The last 5 minutes or so aim to blow the doors off heaven itself and sound like Mahler harnessed all the starlight and turned it into the voices of all the Gods. It's one hell of a jam session. (A good overview, including translations, here.)
