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

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