Are we talking good value headphones, or just best for dirt cheap?
Monoprice 8323 is best for dirt cheap. I think the JVC S500 is the current "omg god tier sound for so cheap omg" headphone.
As for value. I think all cheap but decent headphones like the 8323 are a lot better value than anything more expensive. They all provide most of the sound for a tiny fraction of the price. Audio quality is something that runs into diminishing returns pretty quickly.
It would be cool if someone were to do a huge under $50 roundup like we have the portable full size round up or the top of the line round up.