Somehow, while pouring in over 20 a game, Duncan nearly managed to rule just as well as Ben Wallace(notes) did. He covered huge bits of floor while his guards recovered after the "screen" part of the screen-and-roll, blocked shots on the ball (he's been especially adept at giving a small guard a small step to drive with his strong hand, then recovering quickly enough to swat the shot with his left hand), and secured rebound after rebound. And he never fouled. Never sent teams to the line for cheap points. That's so huge.
As well-regarded as he is, Duncan is still underrated defensively. You could argue for him to be higher on this list, as well. It would be a tough argument to counter.
http://sports.yahoo.com/nba/blog/ball_dont_lie/post/The-top-10-best-defenders-of-the-last-decade;_ylt=AguWIobfTvquo7.Qzi8LTF.8vLYF?urn=nba,185792