Tom Ziller is ranking the top 50 players in the league. He puts KG at #4. The rest of the top five are Duncan (5), Kobe (3), Chris Paul (2), and one has to assume LeBron at #1 (but that post isn't up yet). And in case you forgot, Paul Pierce was in at #17 (behind guys like Boozer, Brand, Billups, and Ginobili).
Regarding Garnett, Ziller says this.
All this begs the question: is Garnett as good a defender as Duncan?
It's a tough case to make, when you consider San Antonio has been at or near the top of defensive rankings for basically the entirety of Duncan's career, while Garnett's Minnesota team fluttered about depending on the personnel. I think 2007-08 in Boston is telling, though. For the first time since Chauncey Billups, Garnett had a point guard who couldn't be described as a matador. Sam Cassell, Troy Hudsen ... could Duncan anchor a defense with marshmallows and still find himself with the No. 1 defense? Give Garnett a Rajon Rondo, it's a better comparison.