Again, Bart Starr played versus weaker opponents due to less parity among teams. As a result, not only did he have more opportunities to make playoffs, but also perform well in them. Would be great if there was some way to adjust QB rating for disparity in competition AND changes in rules among different eras. Unfortunately no such thing exists; at least none that I know of. QB's today may face greater competition, but they also generate better stats because receivers cannot be checked all the way down the field. As a result, Greatest of all time is mostly a matter of opinion.
EDIT: Mark Sanchez is number 6 on that list