My choice are 'Homage to the mountain' by Yello.
'Waiting for Cousteau' by Jean Michel Jarre.
'The end' by The Doors.
'Requiem' by W G Mozart.
anything by Bjork
The 1812 Overture. No, only kidding. The second movement of Mozart's Piano Concerton no. 21 (popularly known as Elvira Madigan, after the film that used it). Beautiful, soothing, relaxing.
Personal fave verrrry difficult to choose... Elvira Madigan, Moonlight Sonata, Air on a G string, the duet from Mozart's Marriage of Figaro (as used in the Shawshank Redemption), Canon by Pachelbel, Puccini's O mio babbino caro... There's just sooo many!!!