Obviously everyone knows how great the Spurs have been defensively. They have by far the #1 defense in the league with a 97.8 DRTG. Most people just stop there and chalk it up to having guys like Kawhi, Duncan, and Green. But the interesting thing is, the Spurs are elite defensively no matter who they throw out there.
For reference: Hawks are 2nd defensively with a 102.1 DRTG, Heat are 5th with a 103.6 DRTG, and the league average DRTG is 106.1
Spurs with Kawhi and Duncan (1059 minutes): DRTG = 94.8
Spurs with Kawhi, no Duncan (983 minutes): DRTG = 98.3
Spurs with Duncan, no Kawhi (195 minutes): DRTG = 98.4
Spurs with Kawhi, no Duncan or Green (536 minutes): DRTG = 96.5
Spurs with Kawhi, no Duncan or Green or Aldridge (210 minutes): DRTG = 94.5
Spurs with no Kawhi or Duncan (979 minutes): DRTG = 101.7
Spurs with no Kawhi or Duncan or Green (609 minutes): DRTG = 103.5
Spurs with no Kawhi or Duncan or Green or Aldridge (514 minutes): DRTG = 103.4
Spurs with Diaw and West (499 minutes): DRTG = 100.5
If you don't get where I'm going with this, pretty much every lineup the Spurs throw out there is elite defensively. Take out 4 of the Spurs' best defenders and they still play defense at a level that would be top 5 in the league. That's absurd. Keep in mind we're talking about lineups featuring Mills, West, Diaw, etc...not all-world defenders.
Throw in the big guns like Kawhi and Duncan, and the defense turns into something the league has never seen. The best defense of all-time (going by lowest DRTG relative to the league average) is the '64 Celtics team with a DRTG 10.8 points better. than the league average. If you look at the best defenses since the 3-point line was introduced, it's the '04 Spurs with a DRTG 8.8 points better than the league average. Spurs with Kawhi and Duncan on the court have a DRTG that's 11.3 points better than league average.
tl;dr: Obviously guys like Kawhi and Duncan deserve a ton of credit for what they're doing...but god damn does it seem like Pop and the other coaches have mastered defense