Charles Ives - The Unanswered Question (performed by the Chicago Symphony under Michael Tilson Thomas)
The full title Ives originally gave the piece was "A Contemplation of a Serious Matter" or "The Unanswered Perennial Question". His biographer Jan Swafford called it "a kind of collage in three distinct layers, roughly coordinated." The three layers involve the scoring for a woodwind quartet, solo trumpet, and offstage string quartet. Each layer has its own tempo and key. Ives himself described the work as a "cosmic landscape" in which the strings represent "the Silences of the Druids—who Know, See and Hear Nothing." The trumpet then asks "The Perennial Question of Existence" and the woodwinds seek "The Invisible Answer", but abandon it in frustration, so that ultimately the question is answered only by the "Silences".
This piece is, in my opinion, one of the greatest achievements of classical music. It was also the piece that inspired Bernstein's outstanding Norton Lectures series of the same name, which talks about musical linguistics, aesthetics, and philosophy and is something that every musician should devote some time to hearing (all six of his Norton lectures can be found on youtube in their entirety!).
The strings are beautiful; the way the trumpet and woodwind quartet alters it is transcendental. It is a haunting piece of music that you will carry with you forever once you have listened to it.