Arm strength is great to have, not that the great QB's have needed it. So while it's an interesting topic, arm strength is just part of the equation.
During the 2011 NFL combine, it was reported that Kap had the strongest arm, just over Mallett, then Newton.
And I've posted this before, but Chip Smith was a trainer for both Kap and Cam Newton. And he said Kap throws harder (I also read in the past that Smith said how Kap's throws "whistle" when he throws them, but can't find the link now). But to me, the velocity stuff is less significant than when Chip Smith said:
Of the 56 quarterbacks I've worked with, Colin is the most prepared for the NFL," Smith said. "I've never seen the kind of complete package like Colin Kaepernick has.