From what I've done: Blood DK > Feral (I suck at feral) > Ret paladin (0 gear might have had something to do with it) > Holy paladin > frost/unholy DK > Vengeance DH
Vengeance is just so much easier than Blood, even if you get hit by a knockback you can just glide back to safety, you do more damage, and have more options to deal with the small adds