What songs do you listen to to test a system's midrange performance?

Sinatra's "Wee Small Hours Of the Morning" album is a mids feast. It's almost cheating the question because it is so good that it sounds at least good on most systems. On an advanced system it is pure pure.

Whenever I think of this album it is as if there are actually two performers that need to be accounted for - Sinatra and the woman to whom he is singing. She is just as present as he is.

Keb’ Mo’s CD Slow Down for male vocals, and “Clifton’s Gold” from Joe Sample’s Old Places Old Faces for sax.

I start with a sampler disc by Analogue Productions that I received when first subscribing to the now defunct Fi magazine.It has a little bit of everything.

Great recommendations, everyone. I've always liked that first song by Brubeck. I haven't heard it for decades because I never knew the name of the artist until now. I see a purchase in the near future. And I love steel string guitar. Along with Frank, I love the well orchestrated music behind Dean Martin and Nancy Wilson. Curtis Mayfield and the Impressions put out some well recorded tunes. But probably my  favorite midrange testers are Alison Kraus and Union Station's "Lonely Runs Both Ways" album and quite a few of the Cowboy Bebop Soundtracks (I was introduced to these by my kids when they were teens). Thanks all. I will definitely be running all these tunes through Clayton Shaw's Caladans when they arrive. And Simonmoon, interesting info on OB's. The Caladans will be shorter and narrower than my DIY OB's.