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

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.

JLP, Al Dimeola, Stanley Clarke - Rites of Strings CD. One of the best produced CDs I've ever heard.

And, of course, Dire Straits Brothers in Arms 45 lp.