I used to use

Floyd - Wish You Were Here : Intro on the guitar takes lots of people by suprise. Hard center image gives some people their first real experience with speaker imaging.

Cranberries - No need to argue : nice sounding female vocal combined with a pipe organ that makes any decent speaker sound impressive compared to the low end client's home speakers.

Then there are the standards or songs that clients have heard repeatedly on other systems and can make analytical comparisons such as David Brubeck - Take 5, Miles Davis - So What (first song off of Kind of Blue), Beethoven - PSno14C#m(Moonlight) - PSno8Cm(Pathetique) (usually go with Rubenstein's "unique" interpretations, Horowitz, or Earl Wild), Tchaikovsky PCno1bm (must go with VanCliburn here, the American that beat out the Russians in Moscow!), Buena Vista Social Club - Chan Chan... just too much to list.