As you know J, for me Manning never had a choker image or can't win the big game image. It was more "Manning never went to the SB, but he easily could have." The farthest I would go was "Manning can't win the big one right up until the point when he wins the big one", which obviously isn't very telling.
That said, where does he rank all-time if he wins? For me, he probably won't change places depending on how he does in one game.
If he never plays another down, he's behind Young, Tarkenton, Unitas, Marino and Montana, in no order. I don't know what best ever means, so depending on your definition he could be behind Staubach too.
With one more "Manning" year next year....basically meeting whatever his projections are...I'd have him ranked just behind Young and Tarkenton in terms of
"value added", which is admittedly just a start. I've got no doubt that Manning will at some point pass Tarkenton for most value added ever as a passer, although he might later in his career regress.
Will he be the best QB of all time? He'll have a pretty spotless resume. The only downside I see is his statistics will arguably be inflated by his supporting talent, scheme and dome play. But I think he'll most likely end his career as the top statistical passer of all time. And in many minds, he'll be the GQTEPBVY.
(Greatest Quarterback That Ever Played Before Vince Young).