What is your favorite piece of clical music? ?

Well, it's finals week here at northwest university and I think my head is going to explode if I have to look at any more calculus. This being said, does anyone have suggestions for clical music to (hopefully) put a mind at ease? If not, just share whatever your favorite piece might be. (mine's either the spring section of Vivaldi's four seasons or beethoven's Moonlight Sonata. :] ) thanks everyone!