Peyton Manning and Tom Brady had one of the greatest quarterback rivalries in NFL history. For close to 15 years, the two faced each other many times in exciting games.Over the years, Manning had the chance to observe Brady many times, and he recently spoke about his observations on "RUSHMORE." Ari Emanuel asked the Broncos legend about Brady:“Can I ask you a question?" he started [02:00]. "When you were playing against him, is there one game, one play that you remember, you're like, wow, the biggest mistake you could ever make?”Manning spoke about the Patriots legend's late-game clutch.“The biggest mistake you could ever make playing against Tom Brady was to score in the fourth quarter and leave him too much time on the clock, because it was just a guaranteed loss," he said. "If you scored with a minute and a half to go, and he had that much time, you were going to lose.”Tom Brady is widely regarded as the most clutch quarterback in NFL history. While he wasn't the most mobile, strongest, or fastest, he was renowned for his intangibles.Among the numerous game-winning drives and comebacks he put together, one game stands out above all: Super Bowl LI.Tom Brady's greatest career comebackSuper Bowl LI is one of those games that are part of NFL lore. Tom Brady and the New England Patriots took on the loaded team of the Atlanta Falcons at NRG Stadium in Houston in 2017.The Falcons shot out of the gates, guns blazing, hanging 21 unanswered points on Bill Belichick's defense. Brady and the offense also couldn't get going, putting up only three points in the first half.The second half also didn't show any promise at first, as Atlanta outscored New England in the third quarter. However, Brady's clutch that Peyton Manning mentioned kicked in midway through the third.The rest was history.Tom Brady went on to pull off one of the biggest comebacks in NFL history, driving the Pats for 31 unanswered points. The Falcons collapsed, as Belichick seemingly figured out Matt Ryan and the offense.Brady's otherworldly performance saw him finish the game with 466 passing yards, two touchdowns and one pick to hoist the Lombardi Trophy.