What are your favourite clical music pieces? Why?

Mahler's Seventh symphony because it's so damn brilliant (as is his First), Rachmaninoff's Second symphony because of the melodies (what else), Dvorak's Ninth because of the melodies as well, Stravinsky's Petrushka because of the odd instrument pairings which generally lead to rather funny yet intriguing colors and sounds, the list goes on. I really can't name all, because this post would be huuuge.