That was clearly Philly’s best defense in 91 but they only went 10-6 and they missed the playoffs. Can’t even come close to making an argument that defense was better than the 85 Bears when they lost 6 games and didn’t even make the playoffs. Most defense rankings don’t even list that defense in the top 10 but a few do. Practically all the top 10 defenses won Super Bowls. No defense was better than the 85 Bears. Their record proves it, and the rankings prove it. I have yet to locate an all-time ranking that doesn’t have the 85 Bears defense ranked number one.
You can make the argument, and Football Outsiders, among others, did in 2017. If the Eagles won the Super Bowl in 1991, they would probably be considered the best defense of all-time. This is Ryan's defense without Ryan, run by Bud Carson.
The Eagles had problems on offense, and that is not on the defense. Rich Kotite may be the worst coach of the past 50 years. Cunningham went down the first game for the season. The O-line sucked balls, one of the worse in the league. They played on more short fields than any team. They had to play, just in the East, the team with the best offensive DVOA (Washington), the fourth (Cowboys), and sixth (Giants) twice. Only the 2002 Bucs have had a better pass DVOA, and the 2000 Raven's a better run DVOA. Teams ran a ridiculous low 2.97 ypc. QB's only completed 44% of their passes against that team. 55 sacks, 43 forced fumbles, and 26 interceptions. That is insane. I also believe they led the league in fewest points, fewest rush yards, and fewest pass yards.