It depends on how you want to define the best QB of all time.
Are you defining them by regular season or post season?
The best teams win in the post season because of the level of competition. Manning has not been on very many teams that would qualify as the best "team". This year like just a couple of others Manning was on a good overall team. But this year when Manning had a good team around him the coach didn't do anything to help him either with the conservative play calling. Wasted time on the clock. A problem with defensive minded HC's.