Demo suggestions:

Jazz: Norah Jones Pretty standard demo disc.
Pop/Dance: Kristine W An unusually well recorded/engineered piece of dance/pop
Country: Johnny Cash You don't have to be a big country fan to appreciate some of the items on this treasure.
Classical: Holst: Planets (This particular recording is pretty amazing.)

and for a simply wonderful recording: Dead Can Dance (track #1 is especially fun)