Classical piano, definitely. I'm a pianist so there's really nothing more appropriate I think. Also sometimes really obscure stuff like stuff you'd hear at a Cirque du Soleil performance.
Usually instrumental music. For example, Nine Inch Nails' Ghosts I-IV used to help me sleep for over a year. Still can. I had some of the best sleep listening to all four Ghosts.
Basically operatic and classical is how I chill out. I throw in some piano-driven downtempo songs, electronic / chillout tracks, and things of that nature depending on my mood, but classical and opera is definitely what I listen to when I'm stressed and really need to relax.