I have not heard the Sony MDR-V6, but I suspect you would prefer it to the HD280 for general purpose listening.
However, for general purpose listening, I suspect you would prefer the HD555 (open cans) for $100 to both of these.
And although I have not heard the RP-21 (closed cans) for $100, I suspect you would prefer it to the Sony or HD280 for general purpose use.
For general purpose use, it's hard for me to say whether you would prefer the HD555 or RP-21. It could be either. The RP-21 probably has deeper bass.