We all saw the level of training DHs undergo, it is described in the quests and the books as the utlimate of both magical and physical capability. The same is not said of the DKs.
And the demons Illidan and his hunters are trained to fight are far more powerful enemies than the living creatures of Azeroth on average, so training to find a demon is tougher job than the foes DKs and others would likely encounter ( i mean in general ofc) - to me that means the DH is ahead of the DK. I'm not even sure the DK is second place, I'd place him 3rd behind the druid.