My favorite quality is the soundstage so any open-back HD800 series headphones (HD800, HD800S, and HD8XX) should be my favorite, also their frequency responses are easily manipulated with EQ.
Named-User-who-died
The validity of my following statement is unproven and may require criticism as I don't have access to various headphones and equalizers.
I imagine someone with OCD could get a headphone with an easily manipulatable frequency response when EQ is applied to emulate any other headphones they want with the exception of the fixed soundstage, transient response, imaging, detail retrieval and perhaps THD, though again, I don't have enough information to confirm the validity of this idea and perhaps I could create a post about this later, but if my statement is valid, I presume based on my current limited headphone knowledge that an HD800 series headphone or a Stax electrostatic (like the Stax SR-L300 + Topping EHA5 for 750$) + EQ and speaker emulation software would be the ultimate performance to cost ratio, they both have a good transient response, detail retrieval, THD level, and an easily manipulatable frequency response curve when EQ is applied, and they can both get excellent soundstage performance when using speaker emulation.
I got 2 mass drop Sennheiser headphones, first the HD6XX and then the HD8XX, with the latter being open-box. Also, I got waterproof swim buds earphones!