Headphones

Anything "classical." Or really, anything recorded live. I was listening to a live recording of Max Richter's On The Nature of Daylight - already a beautiful piece in and of itself, but the recording had an enormous bass rumble that echoed throughout the venue.

Acoustic stuff like bluegrass, folk, rando singer with a guitar or piano, etc. all sound spectacular. Post rock stuff, like Sigur Ros, God Is An Astronaut, Explosions In The Sky, are perfectly suited as well. I'd argue that even stuff unsuited for the HD800S will still sound great. I listen to a lot of metal, punk, rock, etc. on them as well, and the sense of space/separation they give to those tracks always puts a smile on my face.