This year's Alabama defense was one of the best ever to take the field of play. OK, I can't really say that because I haven't studied the entire history of college football defenses. It is certainly one of the best of its era, which probably then puts it up in the top of all time.
Let's go over some of the numbers, shall we?
All-Time History
ESPN's Brad Edwards put out a couple of remarkable numbers earlier this week from the NCAA's stat archive that stretches back to 1937. First, Alabama is only the second team to finish first in rushing, passing, and total defense after 1986 Oklahoma. I'm assuming those are on a per-game basis. Second, the 77.9 yards per game gap between No. 1 Alabama and No. 2 LSU in total defense is the largest ever. Wow.
Scoring Defense
Unfortunately, the NCAA doesn't separate out special teams and defensive scores from its scoring defense stats. Keep that in mind here. Plus, remember that the clock rules in 2006 and 2008-present have resulted in faster games and have an effect on points scored per game.
That said, only four teams have allowed fewer than 10 points per game since 2000: 2011 Alabama (8.15), 2008 USC (9.0), 2001 Miami (9.4), and 2000 TCU (9.6). If you take out Bama's game against I-AA Georgia Southern, it falls to just 7.1 points per game.
Total Defense
This year's Alabama team was one of only two since 2000 to allow less than 3.5 yards per play. It allowed 3.32 per play, while 2004 NC State allowed 3.47 yards per play.
Passing Defense
This year's Crimson Tide defense allowed just 4.34 yards per pass attempt. That is the fewest since 2000, just ahead of 2002 Miami (4.41). It is second since 2000 in pass efficiency defense at 83.69, behind only 2001 Miami's mark of 75.60.
Only six teams have had both a defensive passing efficiency below 90 and fewer than five yards per attempt allowed: 2001 Texas, 2002 Miami, 2006 Wisconsin, 2008 USC, 2011 Alabama, and 2011 South Carolina.
Rushing Defense
I prefer to keep sack statistics out of rushing totals; the NCAA does not. The NCAA's reliable sack statistics go back to 2005, so that's as far back as I can go with this.
The best rushing defenses since '05 in yards per carry were 2008 TCU (2.98), 2010 Boston College (3.172), 2005 Kansas (3.174), 2007 Boston College (3.177), 2007 Oregon State (3.189), and 2011 Alabama (3.19).
However, things change if you take out I-AA data. Do that and '11 Bama goes to the top. At 2.63 yards per rush allowed, it's the only defense to allow fewer than three yards per carry since '05.
Third Down Defense
The NCAA's data on third down defense goes back to 2005 as well. This year's Alabama defense had the third-lowest third down percentage at 24.46%, behind only 2010 TCU (24.10%) and 2005 UConn (24.20%). Those three teams were the only ones to allow conversions on fewer than a quarter of third downs.
Tackles for Loss Percentage
As an attempt to come up with a pace-neutral tackles for loss stat, I divided teams' total tackles for loss by total plays defended. As with sacks, the NCAA's TFL stats go back to 2005.
This year's Alabama team comes in tenth since then at 13.33% of plays resulting in a tackle for loss. Only 16 teams since '05 have recorded a percentage at or above 13%.
Overall
You'll notice that Alabama is at or near the top of each of these categories. Only a couple other teams even show up in more than one. Based on looking over all the stats I compiled (including some not posted here), 2011 Alabama was clearly the best defense since 2005. Only 2008 USC and 2008 TCU are really in the same neighborhood. I'm limited on what I can say about defenses from before 2005 because of the lack of availability of data, but 2001 Miami is right up there with '11 Bama.
I can say with relative certainty that this was Nick Saban's best individual defense. The only time any of his LSU defenses comes up based on my criteria was the 2003 defense in passing efficiency (89.81). The only year other than 2011 that an Alabama defense appeared based on my criteria was 2005 (10.7 points per game allowed). I haven't studied any of his Michigan State or Miami defenses, but I can't imagine them being better than this one.
All in all, it was a season for the ages for the 2011 Alabama defense.