...take 1 CD LP or 1 digital music recording to test a new audiophile friend's system...

Ketty Lester, Love Letters 1962 vinyl

Tonian Trio II Moon Jazz ...this is Brad Dutz, John Fumo and Randy Landas. Recorded, mixed and mastered by Tony Minasian... this is a stunning recording by a top top and rather different approach engineer, showing off what Red Book can really do ...so if its truly only 1 CD for me its this one ..

Unless you are into hard rock or fusion style jazz, I would take Alison Krauss and Union Station live CD. Stunning SQ, lots of all the audiophile qualities we strive for. Musicianship is superb and her voice will test many a system for purity. SACD is even better.

I don't bring anything to "test" anyone's anything. I listen to what they have. Only times I ever brought anything was either a) way back in the beginning when I was still wet behind the ears clueless or b) once I had been a few times, knew what the guy had, and thought people would appreciate hearing something new.  

They did. 

That's what its all about: the music. Not checking off tick boxes some other dude thinks matter.

Years ago I made a CD-R of a variety of selected tracks from different recordings, each of which highlighted a particular aspect that I was interested to hear.  Much easier than carting around a stack of CDs from store to store and feeding disks in and out of the machine.