The Alabama defense has been brilliant so far this year. Let's count the ways in which the defense ranks highly. All of these categories leave out stats from games against I-AA opponents.
Scoring Defense
This, of course, is not entirely on the defense; the NCAA counts special teams and defensive touchdowns allowed. Take it with a grain of salt, but consider that Bama is leading the country at seven points per game allowed. The next team on the list is Wisconsin, who is about a field goal behind at 9.6 per game. You read that right: Alabama allows the fewest points per game in the country by a field goal.
Total Defense
For this, let's look at yards per play allowed; it evens out issues arising from teams facing differing numbers of plays per game. Alabama is tied for first with Michigan State at 3.2 yards per play; third place is Penn State at 4.1 per play. Bama is tied for first, and the lead over second place is just shy of a full yard per play.
Rushing Defense
Would you believe the Crimson Tide is first in this category too? Because it's first in this category too. With sacks not included, Bama allows 2.19 yards per rush. Second place is Cincinnati at 2.92, about three quarters of a yard behind.
Passing Defense
For this I use passing efficiency. It's not a perfect stat, but it's one that correlates highly with winning percentage. Bama is again tied with Michigan State for the lead at 84.91, with third place being Penn State at 90.37. Do you prefer yards per attempt? Bama is alone at the top at 4.5, with the Spartans 0.1 behind.
Sacks
Ha! We finally found the weakness, because Bama ranks only 79th nationally in sack percentage (sacks divided by the sum of pass attempts and sacks). Of course if you asked Nick Saban about that fact, he would probably respond with a snarky answer including one of his favorite swear words. He doesn't really care about sacks. So while Bama isn't prolific at sacking the quarterback, the head coach could give a, well, you know
Tackles for Loss
I'm going percentage on this one as well, with TFL percentage being TFLs divided by total plays defended. In this measure, Bama is second nationally at 13.97%, behind only Michigan State at 14.65%.
Third Down Defense
Alabama is eighth nationally here, allowing third down conversions just 28.04% of the time.
Red Zone Defense
The Tide allows opponents in the red zone to score just two thirds of the time, good for a tie at seventh nationally. It only allows 1.28 red zone trips per game, good for second nationally behind Michigan State, but I don't have total drive numbers to truly calibrate that figure well.
Explosive Plays
Alabama is first nationally in the percentage of plays that end up going for 20+ yards at 3.19%. It's third nationally in explosive rushing plays allowed at a rate of only 0.59%, though it's tied for first in raw numbers having allowed just one (and it was in garbage time at that). For passing explosive plays, Bama is fourth nationally at 5.04%.
Wrap Up
In these 11 categories, Alabama is top ten-ranked in ten of them. The next-highest team in number of top ten ranks is Michigan State with eight, followed by LSU with seven. Plus, the one category Bama is not top ten in is sacks, which Saban doesn't believe is a noteworthy stat. The Tide is first overall in five of the categories, with the next highest being Michigan State again with four.
Also consider that Bama has been excelling over a larger set of games. It has played seven I-A opponents, compared to Michigan State's five. In addition, Alabama has played the 31st-ranked schedule according to Jeff Sagarin, whereas the Spartans have played the 84th-ranked schedule. Bama has won every game comfortably too, so it has a lot more garbage time baked into these stats than MSU (who has a loss and a close win in those five games) does. LSU, notably, has played the 10th-ranked schedule.
It's impossible to know yet where this defense will end up historically, but it's on pace to be one of the best in recent memory. If it keeps going as it has been going, its peers won't be anyone from this year but rather 2001 Miami (FL), 2008 USC, and 2009 Alabama.