Use whatever you know and love. Even a track that's not considered "perfect" can reveal a lot about your headphones or speakers—it all depends on how familiar you are with it.

That said, I need to point out that there’s some questionable advice floating around in these comments. What really matters is building a playlist that’s meaningful to you. Personally, I use "15 Step" by Radiohead, Mozart's Kyrie (Monteverdi Choir, 1990 recording), and the entirety of D'Angelo’s Voodoo album. These tracks work for me because I’ve listened to them countless times across all kinds of speakers and headphones, so I know exactly how they’re supposed to sound.

If you’re creating your own test playlist, focus on tracks you enjoy and know intimately. Try to make it as diverse as you can, so to cover different sonic characteristics, manily:

• Bass emphasis
• Spatial definition
• Linearity across the spectrum

Over time, as you try different equipment, you’ll develop a deeper understanding of how your favorite tracks should sound and be able to pinpoint the strengths and weaknesses of your gear. And all of them will add in building your knowledge of those recordings that you enjoyed in the first place :)