which char to focus on for PVP and Why?

thanks for the info shinku but I don't want to open a new char to PVP, just focus on one of the ones I already have.
not interested in bandwagoning for example marksman ( or whatever will be no.1 ) just to have easy mode PVP. looking for the best choice out of what I already have.

if you have a tespia invite, why not try them there? and i only explained my opinion about dark knight, because it's the class i know how to play and with which i PVP's in tespia, others can add about the other classes and youcan make your own ideas, i just said that about mechs and BW's because as a dark knight, they annoyed the crap out of me <.<