What about the '08 Celtics or the '74 Celtics ?
Garnett's '08 team, with Thibs coaching them, was the best team man-to-man I can remember, That group suffocated their opponent by overloading the ball side fearlessly. Their defensive teamwork, where the help-side defenders essentially played on a string together, allowed them to relentlessly pressure the ball.
Cowen's '74 champions should also be mentioned, I would think. Just look at the individual super-defenders, Don Chaney, John Havlicek, Dave Cowens and Paul Silas along with a steady JoJo White. Very formidable group who played well together on defense. It was this defense that smothered Oscar Robertson, forced turnovers and frustrated Jabbar in Game 7 that won us our first post-Russell title.