It's hard to compare eras, even the span of 1960 to now. So I guess if you say "relative to the era they played in, and the rules by which they played..." this is the best defense. In a vacuum, how would the 1962 Packer defense do vs. the 2000 Rams offense? Impossible to say, really.

The most dominant defenses that I've ever seen are the 1985 Bears and the 2000 Ravens. They were both nasty.