I went back and looked at data dating back to the the merger in 1969. That's 46 NFL seasons. I looked at how the best defense in the league did in terms of yards allowed per play (ypp) relative to the league average. In 1977 the average NFL play gained 4.41 yards (lowest in the history of the league). In 2015 the average NFL play gained was 5.48 yards (highest in the history of the league.
By normalizing to league average you get an idea of how good any team was relative to the rest of the league that year. For example a team allowing 4.6 yards per play in 1997 is no where near as impressive as a team allowing 4.6 ypp in 2015.
So here is the data for the league leader in every season and a %difference showing how much better than the average defense that defense was during that season.
The Broncos defense has only led the league in ypp allowed twice during the history of the NFL, in 2012 and again this past season. The defense that lead the league but was the closest to league average was the 1993 Green Bay Packers. They allowed 4.49 ypp with a league average of 4.94. Only six teams have ever lead the league in ypp allowed and won the super bowl in that same year, but two out of the last three super bowls were won by said teams. I have italicized those teams in the table. Note that the 1985 Bears did not lead the league in ypp, nor did the 2000 Ravens.
The top 10 defenses all time in terms of performance above average in ypp are shown below.
Notice that the 2015 Broncos are 6th on this list. Also note that this list only includes the league leader in ypp, there were some other teams that were second in the league in ypp allowed in a given year who were close to making this list, but none did better than 1971 Minnesota Vikings who finished second to the Baltimore Colts but still only allowed 3.88 ypp in a year when the league average was 4.74 (18.3%).
What the 1970 Vikings defense was able to accomplish in terms of ypp will probably never be matched. They allowed 2803 yards on 814 plays (3.44 ypp). The league average that year was 4.64 ypp - meaning that they allowed exactly 1.2 yards per play less than the average NFL defense. That 1991 Eagles defense was also pretty impressive (Reggie White in his prime as minister of defense). They allowed 3.92 ypp in a year when the league average was 5.05.
We only have three teams on the top 10 list from the post-rule change era (rules changes to favor the offense started right around 2000 when you really start to see an increase in ypp league-wide (see the bar graph above). The great Ravens' defenses in 1999-2006 had only one team on this list (in 1971 the Baltimore franchise was the team currently in Indianapolis - the Colts).
The 2002 Bucs defense gets mentioned in the list of "greatest defenses ever" when NFL fans talk defense. They allowed 4.20 ypp and the league average was 5.16 that season. The 2009 Jets defense is often forgotten because the 2009 Jets lost 30-17 in the AFCG to the Colts - allowing the Colts to score 24 unanswered points. That Jets team had a great defense (despite their failure in the AFCG). They only allowed 4.24 ypp when the league average was 5.33 that year. Finishing the regular season at more than 1 yard better than league average is very impressive.
That brings us to the Denver Broncos 2015 defense. Our defense this year allowed 4.39 ypp with the league average being 5.48. On an absolute basis that 1.09 ypp difference is one of the largest in the history of the league.
It's also interesting to note that the 2015 Broncos defense faced significantly more plays per game than many of the other defenses in the top 10. The 2009 Jets defense was on the field for 953 plays . The 2013 Seahawks defense (not in the top 10) was on the field for 990 plays. The 2002 Bucs' defense was on the field for 963 plays. The 1999 Ravens' defense was on the field for 1040 plays. The 2015 Broncos' defense was on the field for 1033 plays (64.6 plays/gm).