San Antonio currently has a DRTG which is +8.3 relative to league average
That's equal to the 1993 Knicks and around ranked 7th. What's more impressive?
Tim Duncan was a part of a better ranked defense. His 2004 Spurs were +8.8 which would be the 3rd greatest ever (going by estimated possessions for the pre-70s.)
If only accounting for actual possessions rather than entirely estimated, the Top 5 defenses ever were:
1. 2004 San Antonio Spurs - (+8.8)
2. 2008 Boston Celtics - (+8.5)
3a. 1993 New York Knicks - (+8.3)
3b. 2016 San Antonio Spurs - (+8.3)
5. 1994 New York Knicks - (+8.0)
Now on the opposite spectrum,
the Warriors have an ORTG of 114.5 which is +8.4 above the league average.
That wouldn't be the GOAT offense either. That title belongs to the 2004 Dallas Mavericks led by Steve Nash and Dirk Nowitzki. The Warriors would be tied for 2nd with...you guessed it, the 2005 Phoenix Suns led by Steve Nash.
Top 5 offenses ever
1. 2004 Dallas Mavericks - (+9.2)
2a. 2005 Phoenix Suns - (+8.4)
2b. 2016 Golden State Warriors - (+8.4)
4a. 1997 Chicago Bulls - (+7.7)
4b. 2002 Dallas Mavericks - (+7.7)
4c. 2010 Phoenix Suns - (+7.7)
Bolded = teams Steve Nash was on