Ideally, something you are familiar with to have a basis for comparison. A great sounding recording does not help you much if you don't have a reference. I usually take with me things I know well. Recently Vienna Teng, Rostropovich on Teldec, Massiva Attack. For imaging, cannot beat Roger Waters Amused to Death, uncanny. You did not tell us your favorite style of music, so the range to choose from is way too wide. Note this has been discussed a lot, so doing a search on it may bring up things that are in your musical area of interest.
