Songs that you know well (specific versions especially), and have listened to with at least one other pair of headphones (so you know what it should sound like).

Any suggested reference track will sound different when listened to for the first time vs after many times and knowing what to expect.

That being said, my typical reference tracks these days are

• Deafheaven's Dream House (Live) from 10 Years Gone
• [Ryuichi Sakamoto's Solari from async] (https://sakamoto.bandcamp.com/track/solari)
• Ichiko Aoba's Porcelain from アダンの風
• My Bloody Valentine's Loomer from Loveless