The league has had only one undefeated team in its modern era, of course: the 1972 Miami Dolphins. Odds are that all four of this year's candidates will trip at some point. How will it happen? Let's take a closer look at how each team got here and where it is most vulnerable.
(Teams are listed based on the likelihood they'll go 16-0 during the regular season based on ESPN's Football Power Index.)
What has made them great: Quarterback Tom Brady's MVP-caliber performance has lifted the entire offense; it is the league's most efficient and least likely to blunder. The Patriots are averaging an NFL-high 3.05 points per possession, having punted just 23 times and having committed three turnovers -- both league lows. Extracting defensive and special-teams scores, the offense is averaging 35.3 points per game, more than six points higher than the next-closest team.
What will make them lose: The Patriots' quality and depth at cornerback, following the offseason departures of Darrelle Revis and Brandon Browner, remains in question. Pittsburgh Steelers receiver Antonio Brown torched nominal No. 1 cornerback Malcolm Butler for nine catches and 133 yards in Week 1, and the Patriots have faced only one top-10 receiver since (Brandon Marshall, Week 7). Opponents have run well on the Patriots and the 4.2 yards they have allowed per carry ranks No. 21. But in a passing league, the Patriots remain most vulnerable in back-end coverage.
What has made them great: The Bengals have one of the NFL's most explosive offenses. Their passing game is producing an average of 7.87 yards per dropback, second highest in the league, and Gio Bernard's average of 5.63 yards per carry ranks third among qualified running backs. Their offense has produced 28.3 points per game, third best in the NFL, despite running the league's 10th-fewest plays (446). The Bengals' pass defense, meanwhile, is the NFL's second best based on opponents' QBR (41.3).
What will make them lose: Quarterback Andy Dalton has had more touchdown passes than turnovers in all but one of the Bengals' games, and the trendy answer here is that he will regress to his mean at some point. Absent that, however, opponents with a strong running attack could control a game against the Bengals. They are allowing 5.01 yards per rush, second worst in the NFL, and their tackling is in question. Running backs are averaging 1.93 yards after contact against them, also the league's second-worst mark. The best way to beat an explosive offense is to keep it off the field.
What has made them great: The Broncos have the best defense in the NFL. Their defensive EPA, an advanced analytics measure of expected points added over the course of the season, is 63.2 -- more than 20 points higher than the league's second-best team. They are allowing an NFL-low 1.1 points per drive, are pressuring opposing quarterbacks at a league-high 37.3 percent rate and have created 17 turnovers, tied for third in the league.
What will make them lose: It sounds sacrilegious given his Hall of Fame career, and in the aftermath of his recent dismantling of the Green Bay Packers, but history suggests the Broncos will pay at some point for quarterback Peyton Manning's elevated mistake total relative to his reduced production. His touchdown-interception ratio remains a league-low 0.64, and 20 quarterbacks have a better QBR than his 52.0. It's rare for a team to go unscathed this long while led by a quarterback with Manning's numbers. According to the Elias Sports Bureau, his minus-4 touchdown-interception differential is the second worst by a 7-0 quarterback since 1950.
What has made them great: The Panthers' defense has been elite with the exception of a second-half fade Monday night against the Indianapolis Colts. It is allowing 4.69 yards per play, second best in the NFL, and has made up for any mistakes by forcing 17 turnovers, tied for third in the league. The Panthers have top-end play on every level, from defensive tackle Kawann Short to linebacker Luke Kuechly to cornerback Josh Norman, and opposing quarterbacks have posted the league's fourth-lowest QBR against them (43.0).
What will make them lose: Cam Newton has generated MVP talk as the quarterback of an undefeated team, and his unique skill set as a power runner is impressive. But his passing remains inconsistent, even in the context of a weak set of receivers; nearly half of his first-half throws Monday night were deemed off target by ESPN Stats & Information video analysis. Newton's overall completion percentage of 54.2 ranks No. 32 in the NFL. It's true that the Panthers have the NFL's fifth-highest drop percentage (5.2), but Newton has either under- or overthrown his receivers on 23.7 percent of his throws -- the third-highest mark in the league. The numbers suggest there will be a game at some point when Newton will fail to make a throw the Panthers need to remain undefeated.