It was the '76 Steelers and it was 29 quarters. I have got to go with Tron on Pittsburgh here.
Other great D's:
'85 Bears - One season, but it was a great one.
'75 Cardinals - Not enough to get them to a championship, but if they had any offense at all, they'd have won a Super Bowl that year.
'86 Giants - LT and the crew brought it on every down in '86.
'73 Dolphins - The No-Name Defense was just that...a bunch of guys who played as a team and shut down everyone in thier path.
'69 Vikings - The Purple People Eaters at thier very best.
Any Browns or Packers defense from 1953-1968.
2002 Bucs. The best example of the 11-man "swarming" defense.