The couple of analyses I read over the last week (from random sports stat nerdy people, which I can try and grab links for if I must) would put them somewhere around the top 10 since the merger. Where they fell depended a lot of which metrics you used, how the were weighted and adjusted and what not.
One of the bigger controversies that came up was whether or not to weight the stats based on the strength of the offenses in the league this year, or how exactly best to do that. The league is more pass-happy than its been in the past, and the rules more skewed towards the offense pass game, making it harder to defend against than it was 20 or 30 years ago. Then again, it's easy to over adjust for that kind of thing, and there's more to the game than just the passing attack.
Really the Hawks gave up several games like the Superbowl. The opponents passing numbers looked good on paper (the record number of completions, for example), but the points weren't there. They forced a lot of turnovers this year, 28 interceptions in 16 games, or something like that. That was more the story of our season than anything else. We'd win something like 20-13, and 10 of those points were the result for the defense or special teams, either directly or due to the good field position they gave the offense to work with off a turnover. It went beyond the usual "lucky interception of a bad pass" and became more like "eventually we will pick something off." Sherman liked to talk (well, duh, I think we all know that now...) about them baiting QBs to throw; it was part of their game plan.
Then you pull out a quarterback like Kapernick who can run and throw, and they'd have trouble. Mobile QBs were a pain to defend against, and just the run game alone gave them fits at times this year. They excelled against the Mannings of the world, who wanted to sit in the pocket and throw against our over-sized secondary. They were the trump card for that kind of offense, designed specifically to beat them in many ways.
If you want to say we're in the top 10 of the last 50 years, I wouldn't bat an eye. But were not as all-around strong as we are engineered to counter a specific offensive style, one that's popular in the league at this time. What the LOB can say, is that they utterly destroyed the best passing attack the sport has ever seen, in the biggest game of the year. Think of it this way the Seattle defense outscored the highest-scoring NFL offense ever, 9 to 8.
They'll certainly be remembered for that even if the numbers aren't there.
Edited, Feb 4th 2014 9:27am by someproteinguy
That monster in the mirror, he just might be you. -Grover