The NFL didn't keep ratings data before 1987, but the 2007 matchup between the Patriots and the Indianapolis Colts is generally regarded as the most-watched regular-season game in league history.
Sunday Night Football is a ratings bonanza in itself: It's been the United States' most-watched prime-time television series every year since 2011, and it often features the highest-rated non-playoff game each season. Nothing grabs the attention of football fans quite like 'America's Team,' Thanksgiving football, and the Brady-Belichick Patriots. The Brady-Belichick Bowl figures to blow that total out of the water. This year's Buccaneers-Dallas Cowboys season opener drew 24.8 million viewers.
