Who has the best defense in the NFL? It's an admittedly complicated question. Depending on what you value most highly, it could have several different answers.

But while there are several teams that would deserve mention as being among the best defenses in the NFL over the last several years, there are really only two that have been consistently in contention for the title of the single best defense in the league: the Seattle Seahawks, and the Denver Broncos

No NFL team has allowed fewer yards or yards per play over the last four years than the Seahawks. The Broncos rank second in both categories. No NFL team has allowed fewer points, fewer first downs, a lower passing rating, or fewer yards per rush attempt. The Broncos rank seventh, seventh, third, and fourth in the same categories. No team has gotten more sacks or generated more pressures (sacks plus hits plus hurries) over those four years than the Broncos. The Seahawks rank in the top 10 in sacks and top five in pressure.

Seattle led the league in scoring defense every season from 2012 through 2015, and finished third in 2016. They led the NFL in yards allowed in 2013 and 2014 before finishing third in 2015 and fifth in 2016. The Broncos finished in the top five in yards allowed during each of the last three seasons. The Seahawks and Broncos are also the only two teams to finish in the No. 1 spot in Football Outsiders' defensive DVOA (Defense-adjusted Value Over Average, a performance efficiency metric that adjusts for down, distance, and opponent) at any time during the last four years. Seattle took the defensive DVOA crown in 2013 and 2014, while the Broncos took it home in both 2015 and 2016. 

If we acknowledge that both teams are contenders for the title of "best defense in football" once again in 2017 (with minimal roster turnover in both places, that seems like a safe bet), who are the teams that might be able to steal the title away from them for the first time in quite a while? Let's run through some of the candidates. 

New England Patriots

Points Allowed Yards Allowed DVOA
Pass Rush Overall
1 8 23 4 16

The defending champs led the NFL in scoring defense last season and finished a strong eighth in yards allowed. Only seven teams allowed fewer yards per play, and nobody let their opponents score less often as a percentage of possessions. (New England opponents turned only 26.8 percent of their drives into points.)

The one relative weakness the Patriots had was their pass defense, and they went out and added two strong components to rectify that issue by signing former Bills cornerback Stephon Gilmore and trading for Kony Ealy up front. Third-round pick Derek Rivers was one of the most athletic edge-rushers in the 2017 draft, and he should be able to help generate more pressure as well. The Patriots will miss the versatility of Jabaal Sheard up front and Logan Ryan in the backfield, and Chris Long was a solid contributor against both the run and the pass, but this is a defense that was already good, added talent, and should be put in even more advantageous situations by an offense that somehow got more explosive this offseason as well. 

Baltimore Ravens

Points Allowed Yards Allowed DVOA
Pass Rush Overall
7 9 10 5 6

One of only two NFL teams to rank in the top 10 in yards allowed and points allowed, as well as pass, rush, and overall defensive DVOA (the other team is next on the list), the Ravens also ranked second in opponents' scoring rate (28.1 percent of drives ended in points) and fifth in turnover percentage (14.1 percent ended in turnovers).

The run defense should continue to be strong, as nose tackle Brandon Williams is one of the best in the league at controlling the defensive front, and he has a quick, agile corps of linebackers flowing to the ball behind him. Adding Tony Jefferson as a complement to Eric Weddle gives the Ravens one of the better safety tandems in football, and though Carr is not a spectacular contributor, the Ravens know he will be on the field every week -- which is more than they could say about a lot of their corners in recent seasons. Humphrey can slide into the secondary as a role player and take on a larger load as the season goes on, and you just know the Ravens will figure out how to get the best out of flex-y pass-rushers in Bowser and Williams.

Add in the coaching of John Harbaugh and Dean Pees and you've got a team that should be firmly in the mix among the best in the league. 

New York Giants

Points Allowed Yards Allowed DVOA
Pass Rush Overall
2 10 3 4 2

The Giants, like the Ravens, finished last season ranked in the top 10 in yards allowed, points allowed, and pass, rush, and overall defensive DVOA. They had arguably the best defensive line in football, with Jason Pierre-Paul and Olivier Vernon wreaking havoc on the edge while Damon Harrison absolutely snuffed out opposing run games.

Harrison's partner on the inside -- Johnathan Hankins -- has moved on, but the Giants drafted Tomlinson in the second round to take his place. They also added Duke Iheanacho to join a safety tandem that established itself last season. Landon Collins was one of the breakout defensive stars in all of football, while Andrew Adams impressed in 13 starts after signing as a street free agent. Along with Darian Thompson, the Giants now have several options next to Collins at the position. And they still have one of the better corner groups in the NFL as well. Janoris Jenkins turned out to be one of the best free-agent signings of 2016 as he put together the best season of his career. Dominique Rodgers-Cromartie continued playing well until he got injured, and Eli Apple showed flashes as a rookie.

The Giants may be subject to the Plexiglass Principle (teams that improve as much as they did from one year to the next tend to suffer regression the following season), but because their improvement came about as a result of mass personnel changes rather than merely improved play from in-house options, they do seem a bit less susceptible to a sudden downturn. 

Minnesota Vikings

Points Allowed Yards Allowed DVOA
Pass Rush Overall
6 3 8 16 9
  • Key Additions: Datone Jones
  • Key Losses: Captain Munnerlyn

The only thing keeping the Vikings from joining the Ravens and Giants in finishing in the top 10 in all five of these categories was their relatively weak run defense, so they went out and added former division rival Datone Jones. Jones finished 12th among 56 players at his position in run stop percentage last season, per Pro Football Focus. The Vikes will miss slot man Captain Munnerlyn, but luckily they've drafted Trae Waynes and Mackensie Alexander the last two years, and those two should be able to pick up some of the slack.

The Vikings looked like the best defense in the league for a while last season until the offense collapsed without both Teddy Bridgewater and Adrian Peterson, and then the D was just under the weight of too much pressure. Armed with young, quick, athletic linebackers and a consistent pass rush, they should be able to tap back into what they had in the first half of the year. If the secondary plays to its potential and the offense doesn't put them in disadvantageous positions quite as often, they will be in the mix for the title of best defense all year.