What is they best album out there for showing off the sound quality of your system.

Looking for the best of the best if you had one what would it be?

I am lucky enough to own all the MFSL 1 Step Ultradisc, they are all outstanding. Personal favorite is Bill Evans Trio - Sunday Night At The Village Vanguard. I am transported to the venue and the sound of Bill Evan playing live when I listen to this LP. At a more affordable price the Rob Wasserman - Duets LP is extremely good sonically and music wise, very enjoyable. Finally one more LP that will convince you just how good vinyl playback has become; Gillian Welch - The Harrow & The Harvest. beautiful music and an amazing recording/pressing.
Dave Grusin Homage to Duke
While not perfect, the recent Prince release 'Piano and a Microphone' is crazy good. 1983 recording - I think originally on cassette!
Both Sides Now by Joni Mitchell (2000). An analogue sounding CD if ever there was one. It can't sound bad on any system, but it will sound deliciously lush on a good one.

Travelogue (2002) is in a similar vein.
Original pressing of Deceptive Bends by 10cc is one of my goto recordings to show off or test aspects of my system.
Here's two that easily come to mind from purchases made this year.....

Beth Hart "Live @ The Royal Albert Hall"
John Hiatt "The Eclipse Sessions"
Two Against Nature - Steely Dan
Blue by Joni Mitchell
Black Dub
Susan Tedeschi - Live at Austin City Limits
Pretty much any of the Steely Dan albums, my favorite is Countdown to Ecstasy.  Also, check out Alice Cooper, especially the first parts of these songs...Billion Dollar Babies, Ballad of Dwight Fry, Go to Hell.  The Who's "Eminence Front" and Kenny Wayne Shepard's "Blue on Black" are also excellent songs for showing off your system.  
If you have full range speakers that reproduce bass pretty well listen to Bela Fleck's "Flight of the Cosmic Hippo." Make sure you remove valuable items from your shelves .  
We been using 
Mfsl Supertramp crime of the century and Alan parsons I Robot 
@irish_tim : Your mention of Eminence Front reminded me that Pete Townshend's album White City is extremely well produced. The song "Hiding Out" is a great one for exercising your system.