Sorry friend, but you can't use just yards per attempt to judge a QB's effectiveness. It doesn't take into consideration pressures, dropped passes, spikes to kill plays or throw away's. If that's what you want to use though, lets take it a little further. Doing a little research, I found these.
Career Y/A
Peyton Manning 7.7
Tom Brady 7.4
Joe Montana 7.5
Brett Favre 7.1
Joe Flacco 6.9
That's 2 Hall of Famers, 2 Future Hall of Famers and 1 "slightly below average / game manager".