Tom Brady's success should be overshadowed by the fact that Cassel stepped into his shoes and did respectively well, now not so much on a crummy team.
Brady is a great QB, I do not argue that.
Brady also has some stellar receiver's, a fantastic offensive line (always overlooked in comparisons), and many or the years a great defense to keep the team even better balanced.
Give some respect to the offensive line when talking about great QBs, yes some QB's can be successful without a good offensive line, but that is a very small amount and likely very inconsistent.
I think it has to be Manning or Brees, where would the Saints be without him? He has made several subpar WR's look like all pros.