Different teams, different times.
Our loyalties skew our perceptions both at the margins and at the extremes. In '90 with the Bad Boys ascendent I watched stretches of their games in the middle of the season where I thought that there had probably never been better team play. It was probably 15-30 games in December and January where they simply battered every opponent, in a similar way. The opposition would quit by the second quarter, you could see it in their body language.
I'm sure if I was to attack the question today I probably wouldn't even find that they had any historically significant stretches from an historical standpoint. But I know what I saw.
The '08 Raven defense isn't remotely close to the '00 one. Put last year's offense with this year's defense and they have a losing record.
It isn't really accurate to call the '08 squad 'more ball-hawking' unless you are simply arguing that more of their success is from turnovers. They've forced 39 turnovers this year, which is very respectable. The '00 team had 51 which is fairly mind-boggling. The '00 team pitched four shutouts and allowed 75 fewer points.
So we will have to settle for the '00 Ravens having the best defense of ever with the '08 one being one of the many runners up. Except, the '00 defense isn't really in that conversation either.
Stuart Chase wrote an article at Pro Football reference discussing some methodology for determining the great defenses. Using two scoring parameters and two yardage parameters he found five teams (the year's Steeler team is the fifth) who led the NFL in all categories. The greatest defense ever may be something of a surprise
We can see that although the '00 defense doesn't particularly stand out historically, this flavor of Ravens has probably had the longest stretch of dominance - well at least since 1960. To me that is their accomplishment. This year's defense is not quite on par with either the '00 or '06 one, relative to the league, but it is still very good. It is very hard and very rare to be able to sustain a great defense over time. The Bears' longevity from '84-'88 was terribly impressive, but not close to the ten year stretch that the current Raven squad has enjoyed.
1969 Vikings: Post-season failures by the team enabled this dynastic defense to fade into oblivion. The ‘69 Vikings went 1-1-1-1 in our four categories; the ‘70 version went 1-1-1-3 and the ‘71 team went 1-1-2-2. The Vikings had a three year stretch that was never matched. If you add their ranking in each season, you get a total of sixteen. The next best stretches are the ‘84-’86 Bears (24), the ‘86-’88 Bears (27), the ‘85-’87 Bears (27), ‘74-’76 Rams (30), ‘62-’64 Packers (33), ‘72-’74 Steelers (33). The closest modern teams are the ‘95-’97 49ers (38), the ‘99-’01 Ravens (43), ‘02-’04 Bucs (50) and ‘04-’06 Ravens (51).
The ‘69 Vikings allowed 133 points, 0.9 OTA/G, 0.96 ANYAA and 3.47 AYPCA. NFL averages were 301, 2.3, 4.70 and 4.62.