A lot of people, including me, would say the best sound is the 4th movement of Scheherazade recorded by Fritz Reiner and the Chicago Symphony on RCA Living Stereo. Two songs that I think are very well recorded and have been using as test/demo tracks for a long time are Spanish Harlem by Rebecca Pidgeon and Girl From Ipanema from Getz and Gilberto. YMMV as always.

I doubt the OP will find consensus on "the best" sounding recordings, but Patricia Barber's Companion album is one of the most amazing live recordings I've ever heard. And the performance is excellent. It's a jazz quintet in a nightclub setting, with Barber on piano/organ/vocals doing a mix of radical reinterpretations of classic rock and pop songs (e.g. "The Beat Goes On," "Black Magic Woman") and a selection of her own tunes. Michael Arnopol's acoustic bass is recorded as well as (maybe better than) any bass I've heard on any studio recording.

This is audiophile showroom demo material.

Man o man.....there are so many great recordings in the generalized classical space.  I find Trevor Pinnock's early 80's recording with the English Concert of Handel's Water Music to be spectacular in every way.  

My favorite awesome sounding jazz recording is Dick Hyman's From the Age of Swing on Reference Recordings.  The red-book CD is phenomenal.  If you have a way to play RR's HRx version, and you can obtain a copy of it, it's stunning--almost unbelievable!

My pop music choice would be the Dire Straits song, Money for Nothing.  When you want to display your system's muscle and ability to play loud and clean, this recording is the perfect choice.  I also love the way the instruments and voices have been placed on the sound stage.  It's a fantastic recording!