Much of the Bears' strength that year was NOT just their terrific defensive players, but a radical defensive scheme by Buddy Ryan that the league had not caught up to. If you notice, however, the league did catch up to it pretty quickly.
The Bears switched to a traditional 4-3 defense immediately after their Super Bowl win in 1985-86, primarily because Buddy Ryan left the team to become the head coach of the Philadelphia Eagles. Defensively, the Eagles were never as dominant as the Bears had been -- mainly because they simply didn't have the same level of talent as Chicago.
Despite their perfect record, I wouldn't even include the 1972 Dolphins among the top ten teams of all time. They played a very weak schedule that year, and were so unimpressive in running the table that they were actually the underdog in the Super Bowl against Washington.