Car appreciation... the aesthetics, the engineering, etc

Not sure if serious; however:

Testing whilst stationary won't tell you much because what you really want to know is how much you can hear above road / wind / engine noise.

You'll need a reasonably prominent upper frequently range that avoids being harsh and unpleasant. Female vocal, orchestral strings, acoustic guitar can test this well.

You'll want a punchy bass, dynamics arguably more important than getting very low frequency. Just pump some 90s techno for this.

Then try radio 4.

Talk Talk - Spirit of Eden
Steely Dan - Aja
Brand New - Science Fiction
Q-Tip - Kamaal the Abstract
James Blake - The colour in anything

All of those albums have strong production with nicely seperated tracks and plenty of range.

And obviously anything by the Spice Girls

Limp Biscuit - Rolling

... I think or one of the other top hit tracks from that album. Lots of different levels plus effects from different directions.

Pink Floyd - Great gig in the sky
Fleetwood Mac - The Chain (obvs)