I honestly don't have a singular song... I have a playlist of songs across various genres that tackle various things like looking for sibilance, bass slam, sub bass rumble, imaging, soundstage and separation and overall tonality and timbre adherence. Some of the songs are:

Ebo Taylor, Pat Thomas, Henrik Schwarz - Eye Nyam Nam 'A' Mensuro (Henrik Schwarz Blend)

Dua Lipa - Future Nostalgia LP

Daft Punk - Contact, Giorgio by Moroder

Osibisa - Sunshine Day, Ojah Awake, The Warrior

Fleetwood Mac - Dreams (2004 Remaster)

Hugh Maskelela - Coal Train (Stimela)

Billie Eilish - Live at the ThirdMan Records Vinyl LP

And so many more @ u/thor_Alf