The Turn of a Friendly card by Alan Parsons Project since I am so familiar with the sound because I have been listening to it from the time I got my first descent stereo in the early 80's and have heard it in every system I ever owned. It allows me to gauge what I am listening to in great detail.
After that I listen to a good recording of a piano, which in my opinion is the most difficult instrument to fool you into sounding live on a recording.