If you had just one song to play on your system to show someone how great it sounds.... what would it be?

Depends upon the source being used. And then whatever I think is good might not be to another listener. My apologies if you think I'm pissing on your question. It's just that I have never tried to show off my system(s), sound wise. They are here for my pleasure, yes precious, my pleasure.
As the Chief mentioned, it should be something familiar to the listener.
In the absence of knowing that, I would play something with startle-the-crap-out-of-you dynamics as I chose for Glenn's speaker video post. Crank it for the opening transient!
Usually when I ask; “What would you like to listen to?” Company may request an older song, or one that’s poorly mastered and doesn’t show what your system is truly capable of. Ill play a song or artist everybody’s heard of. Michael Jackson~Thriller album would be my vinyl pick. I would use a lossless Hi-Res track initially though for first impressions since they would have the most dynamic range.Similar... I always ask what their favorite song/band is.
an older song, or one that’s poorly mastered
No I didn’t mean imply that all older songs are recorded poorly, but an older 1960s mono album is definitely not going to have the same dynamics as an newer analog album or one digitally mastered 24/192. Newer master recordings would have been recorded/pressed with modern state of the art equipment.So all older songs are recorded poorly? The second half makes sense