If you check out Football Outsiders’ historical DVOA Estimates from 2014, one team sits at the top, and it’s not even close. The second-best team (the 1969 Minnesota Vikings) had an impact of -32.9% DVOA, with the negative number indicating more or less how heavily the those VIkings undermined the offenses that they faced. The number one team? The 1991 Philadelphia Eagles, sitting at -42.4%. In fact, on a year-by-year basis from 1986 to 1991, the Eagles never leave the top five for defensive efficiency.
Over that same time, the Bears fall from first (1986) to eleventh, and while they have some peaks (second in 1990), they also have some valleys (seventeenth in 1989).