Kareem averaged 4.1 blocks and 17 rebounds per game in '75-'76.
Ewing's best averages were 4 blocks ('89-'90) and 12 rebounds ('92-'93) per game.
Wilt averaged 27.2 boards in '60-'61. We'll never know how many blocks per game he averaged, but it's estimated by some to be 6 or more.
Compare to career best 4.5 blocks ('91-'92) and 13 boards ('90-'91) for Robinson and 2.9 blocks and 13 boards for Duncan in '02-'03.
Now Robinson, if considered in the company of centers, is clearly among the best in terms of defensive stats. Duncan, when compared with people who play the game like he does, is not very impressive. What's even more interesting is that his rebounding and blocked shot stats haven't increased since DRob retired.
Kareem was an amazing defender, especially early in his career. Some consider Ewing to be the best defensive center of the modern era (even though I don't really agree with that). I'm sure a lot of people will disagree with Wilt, so here's my justification. I know a lot of people like to play the "that was a different era" card, and for the sake of this discussion I'll grant that it made a big difference. But Wilt still averaged more that double Duncan's career best. I don't care what era he played in, he was a better rebounder than Duncan, especially in terms of desire. He didn't just stick his long ass arms up and catch every shot that careened off the rim like Duncan does. He used to fight mother ers for the boards, which is why he could outrebound other players of similar height (like Darrall Imhoff, for one). One-on-one against Russell, he always did better on the boards and limited Russell's offensive production (most of the time).
And far as Hakeem goes, good jump-shooting does not a power forward make. Just because you can shoot doesn't mean you're a pf. Take Brad Miller for instance, or Sabonis. Hakeem played back to the basket against most defenders, but against taller, longer guys who could block shots (i.e. Ewing and Robinson), he stepped outside and made quick moves to the hoop or shot jumpers. Granted, he slowed down latter on and relied more on his jumper, but early in his career he was a prototypical center (whatever that means).