This is a very vexing question, but one that is great to kick around. I played football (LB and OL) for 14 years and coached for about a dozen and have seen many games by all of the QBs discussed above.
First thing first is that it is impossible, and perhaps unfair to compare all of the Superbowl era QBs together. When the SB era started and until 1977 (I think) they only played 14 regular season games.
Perhaps more importantly the passing rules were greatly liberalized around 1978 or so. The passed the Mel Blount rule that prohibited downfield contact and changed the rules for holding on the O-line that made it a great deal easier to pass block. It is no surprise that almost all of the names mentioned played in the 16 game, liberalized rules era. The game has really changed.
But having thought about it I could put together a list of QBs that I think are truly elite, having seen them all play at least 20 games. They would include:
Bradshaw
Staughbuck (sp?)
Marino
Brady
Manning
Elway
Young
I think Warren Moon is on the edge of the group, as is Tarkenten, but I did not see Tark enough to come to a good conclusion about him. The only reason Fouts is not on my list is that I think some mobility is required to make the greatest all time list.
There are some guys playing right now who we might be able to add to the list, but it is too soon to say.
You will notice Montana is not on the list. I watched him play for years, and despite his success I never saw him as being really talented. He was to my mind a real dink and dunk QB and never impressed me.
Statistics are useful but for a number of reasons quite limiting. And personally I think the NFL passer rating system is deeply flawed.
Now picking the all time best out of that list is almost impossible for me.