I play a survival hunter. I do really well against most melee classes including DK's; In fact, DKs are one of the classes I enjoy fighting, cause most are facerollers with no skill and / or have no idea how their class works.
As a hunter, you can take away the most important thing of any melee class, mobility. Concussion shot, then just keep kiting and its over. Not hard. DK's, sure they have range, but there are many ways around that as well. Rouges on the other hand, is a little bit harder but be sure to spam flare.
The only classes I really fear are paladins and mages. Mages have ridiculous ranged damage, with OP shields to boot.
At the end of the day, there isn't any "best" pvp class, every class is weak against some classes and strong over others. It's knowing how to handle the more difficult classes to fight and learning their play style as much as possible so you can try and counter it.
If your aim is to be the best pvp player in every battleground, then create a hunter, twink your toon out with heirloomed gear, and play low level battlegrounds.