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.
Last night we listened to Donald Fagan The Nightfly.  Awesome!  I have so many albums that sound fantastic, it's hard to pick one.  Dire Straits - all of them - also come to mind.
There are some great soundtracks with superb sound:

American Hustle

As well as many others
Dead Can Dance Into The Labyrinth (new pressings). I think my system sounds pretty good (for mid-fi at best) given its a mish mash of stuff waiting to be replaced (total cost $3300) but this record just........holy crap!

@soundermn : If you like the SQ of Dire Straits stuff (and I do too), check out some of Knopfler's solo work. He has done a lot. I don't particularly care for a lot of it, but Tracker is exceptional. One of my favorite albums. Get Lucky is good and the first CD of Privateering is also good. 

Tracker would be my second choice for showing off my system after Steely Dan's Two Against Nature. I like the music on Tracker better but Two Against Nature may be the best produced album I've ever heard. Even better than Aja and Gaucho.
Almost any direct to disc record
"Wandering" by Yosi Horikawa. I heard it at AXPONA 2019 in one of the Wilson Audio rooms. The first track "Bubbles" is 3D ear candy. This EP doesn't touch all the audiophile performance bases, but it sure is fun. If you like percussion and sound effects, this is the ticket.  
@n80 thanks!  I will check out Tracker and Two Against Nature.  I'm with you that some of the Knopfler solo work is good, and some not so good.  Steely Dan is always great!  
The Gifted Ones - Gillespie, Basie, Brown, Roker. (Pablo 1981)
Buddy Tate, Al Grey  - Just Jazz (Uptown 1984)
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The Gifted Ones sounds great!  Thank you fir that and the many others!
If your system can make simply recorded mono records from the ’50s and ’60s sound realistic and lifesize, that’s really showing off. For example, Question Mark & The Mysterians "Action" LP, side 1 track 3.
Mike Oldfield - Amarok HDCD

It's like having 1000 songs crammed into a one hour long song. It's got dynamics, slam, depth, nuance and emotion.

Other than that I like:  Dr. Chesky's Magnificent, Fabulous, Absurd, & Insane Musical Show on SACD.

For Vinyl I like Dire Straights Love over Gold - Dynamic!
+1 for Two Against Nature as far as an album that can truly make a system sound great. It does have a bit of a sterile, digital sound to it but it is pretty incredibly recorded and the dynamics are awesome. 
Morning Phase and Sea Change by Beck are pretty hard to beat, great albums, great recordings, great productions. They are top of my list along with The Wall and Wish You Were Here by Floyd. 
"Portraits" by Clark Terry SACD on Chesky.  Great jazz quartet with totally open soundstage with literally wall to wall imaging.  An immersive experience listening to top quality jazz.
Lee Ritenour - Wes Bound
Lyle Lovett  - Joshua Judges Ruth 
Acoustic Alchemy - The New Edge
Bela Fleck - Flight of the Cosmic Hippo
Peter Gabriel - Us

+1 Prince - Piano and a Microphone

also latter day Prince records: 3121, Musicology, etc. Prince was an incredible producer. 

+1 Peter Gabriel - Us

Phil Collins - No Jacket Required

Vampire Weekend - Modern Vampires of the City

Fiona Apple - Tidal and The Idler Wheel...
White Winds  - Brothership - Andreas VollenWeider
Probably has sold more high end equipment than anything out there.

I like the early MFSL releases, particularly those that were mastered by Stan Ricker:

Another one that comes to mind is Sting's The Soul Cages CD recorded in the short lived Q Sound.
Party Ball by Stan Ridgway on cd or Vinyl ia a great recording for showing off your system. 
As many here, I've a cadre of MFSL, Three Blind Mice, Chesky, VTL, Opus 3, RR, etc.  But going against all of these fine recordings is Gail Pettis "May I Come In?".  So well mastered, so intimate and a great talent with terrific support.
Also, Pepe Habichuela and Dave Holland "Hands".
@celtic66 Thank you so much for mentioning the Gail Pettis album, "May I come in?" It is terrific! It is a very intimate recording in that it is as if they are performing for you in your listening room. Gail has a great voice and knows how to tell a story with a song. This album is becoming hard to get, so don’t wait! Get it now. It just arrived today and I’m listening to it for the third time! I owe you one @celtic66
Lyn Stanley. any of her albums but especially her new one, "London Calling". masterfully engineered for showing off your system.
You know, now that you mention “London Calling” I would say “London Calling” by The Clash. Pretty much any system can replicate simple music with vocals and a few instruments. The Clash is busy, noisy, pedal to the metal music that is quite demanding when played briskly. Many audiophile systems need not apply.