This will vary depending on who you ask. My own personal favorite in this category would have to be Strauss' Also Sprach Zarathustra. Though the opening fanfare of this piece is oft-parodied and imitated thanks to 2001: A Space Odyssey, the rest is really rather beautiful too. The second movement, in particular, is one of the most beautiful pieces of music I have ever heard. Let alone of the most beautiful pieces of classical music I have heard.