Stereo test I swear the bw2 ost is the ost that plays with the channels the most. This track goes to EXTREME, but it can get crazy with constant channel switching such as this one
Most of the earthbound ost for bass test because I realised with my 58x that it’s a relatively bassy ost, but my favorite track notably gets A LOT better with bass and it’s sanctuary guardian which will constantly thump in the background.
I have many instruments separation test tracks, but the most notable one is this one from the second paper mario game because it’s very catchy and it comes from my favorite ost so i heard it MANY times.
For soundstage test, this one is odd, but when I listened to it on my hd 598 (which really was my first good headphone), I realised the soundstage went WAAAAAY out there so I am curious how my stuff sounds with it since. I still don’t exactly know why a NES song would do this effect, but it is neat to listen to on a good song! Honorable mention goes to the original flatzone song from ssbm
One REALLY good stress test song for whether my gears has murderous highs or not is Maridia from super metroid which first off, it’s kind of picky in terms of quality because if you research this song and you end up on a bad rip, you WILL notice it (the one I linked seemed decent). The main thing with this song however is the highs are EVER SO CLOSE to murder, but they don’t really reach it so a good headphone would reproduce the clarity very well. One honorable mention which I like because it’s one of the simplest track to exist is drought from pokemon gen 3 This one, you might notice sounds harsh on this link and it’s because I can’t find a good rip on youtube, the ONLY version I didn’t find harsh is the ALAC one from itunes that you can buy. So it’s picky AND has very high notes.
For overall test (so just a song to test a lot at once), the classic theme from bejeweled 3 (very underrated ost btw) is my favorite because it lasts for 9 minutes and has multiple phases with different mood in each of them.
Though imo, the best test you can give to your current gear is a long listening session to the majority of your library.