I think he's the greatest regular season QB there ever was. But I rank Brady higher than Manning in terms of all-time great QBs because of what he did in the post season. Had just had a couple of plays went against the Giants in those Superbowls I doubt there would be any debate on the matter.
But to Prog's point, the bad Denver D does deserve its share of the blame. Denver did put up a lot of points. But the worst thing is both the secondary and Peyton failing in the clutch. Really surprised me they didn't try to get into field goal range at the end of regulation--35 seconds, 2 timeouts, no timeouts for the other team. If Fox trusted Peyton, he should have given him a couple of chances to get the ball down the field (or to take a sack/throw it out of bounds if nothing was there). They still could have kneeled on 3rd down and went into OT if need-be.