I think anything under 200 points allowed in a season in this era has got to be considered remarkable. Whether or not it's the "BEST" of course depends on how the postseason plays out. Those 2000 Titans are lost to time and never brought up, but they allowed just 1.6 more points per game than the record holders, then held said record holders to just 134 yards of offense in a playoff game, but lost anyway because of defense and ST scores in an epic battle involving the following coaches: Jeff Fisher, Brian Billick, Marvin Lewis, Rex Ryan, Jack Del Rio, Gregg Williams, Mike Munchak, and Jim Schwartz. Talk about loaded with complicated legacies!
Since the '00 Ravens set this record (Titans also under 200 that year), only the 2002 Bucs have allowed under 200. A Ravens team ('06) and a couple Bears teams ('01, '05) have come within a handful of points, but the last of those was 13 years ago. If NE ended up at 180/190 PA in an era where overall scoring has gone up like 3ppg in the last 20 years, then comparatively that is even more impressive. Whether they're remembered like the 2000 Titans or Ravens depends on what they do in Jan/Feb.