This is a strange thread. To say best ball carrying backs is strange, as all backs should be good ball carriers. Do you mean most dangerous back with ball in hand? Don't know if there's a way to answer it, as wingers are dangerous given space on the wing, and centres are dangerous in midfield, it's not a one size fits all.
In terms of forwards, it's different again. Croft, Messam, Vito, Spies, Kankowski and Higginbotham are probably quicker than many backs when given plenty of room to move can be very dangerous. Read, O'Brien, Heaslip, Burger, Harinordoquy, Tonga'uiha, Parisse and Thomas Waldrom all make a lot of metres per game, the latter of which made the forth most metres in the Premiership behind four fullbacks. The question doesn't really make much sense.