What are the "Must have" LP's?

I have recently re-discovered the LP. I listen to Jazz, blues, classical, and some rock. I was curious as to what my fellow audiophiles here on audiogon believe to be the albums that you simply MUST HAVE. As a sub-question, What LP's do you use to show off your system? Since I am just getting started, I have no real opinions yet, but as I become more and more familiar I will post my findings. Thanks.