That's a hard one as I have lots and I am trying to think of which one I favour most. I have one that is a nature one which has just a subtle hint of music in it. It has rain, storms, the sea, frogs and birds. I think it might be one of the Naturescape CDs. Also, musically, I love Medicine Woman by Medwyn Goodall.
I like Pacobels Ocean. This was a relaxation CD I got from Readers Digest, but of course it is a famous piece of classical music as well. I do have a relaxation CD by a variety of different artists, which was given to me, but I haven't listened to it for a while, so can't recall a favourite piece