Favorite song to demo or test drive speakers?

Some good ones already mentioned. A few odd ones that I used over the years. Pink Floyd Dogs
Depeche Mode Personal Jesus acoustic version
John Hiatt Feels Like Rain
Any drums recorded in the front hallway at Headly Grange . The best sound not recorded by John Bonham may be can’t get enough from bad Company’s debut record

Whenever I make a significant change I play Dave Brubeck & Tony Bennett live performance for president Kennedy in 1962. It touches all the bases. You are there, wonderful recording.

Grace Jones -Slave to the Rhythm 
Eagles - Hotel California (Hell freezes over Album).
Queen - Bohemian Rhapsody

Tom Petty Wildflowers (original vinyl)

Aaron Neville "Everybody Plays the Fool" (if the system has issues with male vocals you'll know it here)
Stevie Wonder "I Believe (When I Fall In Love)" (For the sub-bass synth line) A real LF workout especially for vinyl 
Jewel "Have a Little Faith In Me" (Or, just take the the whole 'Phenomenon' Soundtrack, almost every cut is reference-worthy)
Jackson Browne The Load Out/Stay (Listen for the feedback in David Lindley's vocal monitors on Stay @ 6:45 - the monitor mixer has them right on the edge, and doesn't panic, he just eases them off a few dB, and saved the whole cut - masterful work) 
Everclear "AM Radio" (If the system can upack the layers in this mix, it can handle anything) And while you're at it listen to their anthemic version of "Brown-Eyed Girl" 
Sonny Rollins "Saxophone Colossus" Any cut - pick one
The Mavericks "All You Ever Do Is Bring Me Down" Eee-hah!!
Charlie Haden & Pat Metheny "Beyond the Missouri Sky" 'Nuf said.
Paul Galbraith "Bach: Sonatas & Partitas" This couldn't be transcribed for a 6-string guitar - the finger positions were impossible - so he built a totally unique 8-string instrument with a second sound chamber, taught himself to play it, then transcribed and then played these 32 pieces. Just Amazing. And will test the midrange purity of any system.