Who is the Best Third Down Running Back in the NFL?

I do think Darren Sproles is effective at that portion of the game. His speed, and his ability to find gaps within the defense is what makes him so valuable to the Chargers. But I am surprised that you didn't mention Peyton Hillis? I think Hillis has been tremendous in his running ability along with breaking through defenders. He's also been helpful at goal line opportunities. He might not be considered your average 3rd back in terms of just being in that role, but he is great at what he does. If not Hillis then I do think Sproles is the best in that group of players you have listed.