That's very subjective and varies person to person.
Some prefer flat and analytical headphones (like HD600's)
Some prefer punchy and energetic headphones (like HE-500's)
And some people prefer head thumbing (more like room shaking) bass in a headphone (like sz2000's)
*cough*
*cough*
And guess what? They all sound good...
But good to who? Whoever bought them.
If you give someone who bought HE-500's HD600's, I guarantee you they would say that the HE's sound better.
And if you give someone who bought sz2000's, HE-500's, they would prefer sz2000's.
But does it make any of these headphones bad?
No, they all sound good, but different kinds of types people prefer different kinds of headphones. There is a reason they bought one pair over another because they bought the one has the characteristics that they're looking for.
Even if you have thousands of different pairs of headphones, no matter what, there will always be one you prefer over the rest. Does it make the rest bad? Absolutely not.
TL;DR
Everyone has different tastes.