Ceri: It's one half about frequently seeing the strong-side backer get pulled in the substitution packages, and one half about there not being any dominate foursomes or threesomes in the 90s. The Cowboys were the dominate team in the league until the mid-nineties. I don't think their linebackers would rate on the all-time list. The Ravens group that you mentioned was excellent.
In the 00s the most dominant, creative defense has been Little Bill's. He's had many versatile talents in his linebacking units over the years. But with the possible exception of Willie McGinest, there isn't the hint of stardom among the group. Underrated, solid, steady, tough. But nothing like having three potential Hall of Famers in the unit like the Giants did.