Really, there just are no can't miss defenders in the league right now. All of them have some holes in their game, and can be exploited by certain matchups, and that makes it so tough to put so much value into that one skill.
I think LeBron/Howard are can't miss defenders relative to their position, when they're engaged they certainly are.
KG/Duncan were too, though now they're too old to be as great. (Remember KG taking on a healthy Yao and dominating?)
I agree that KG and Duncan were in their prime. So was Ben Wallace and Ron Artest.
Right now, I think you have Lebron, who sometimes picks and chooses when he will play defense. You have Howard, who can be that dominant defender, but can get a little too into posing for the cameras at times, rather than making the smart defensive play. And then you have some young guys like Sefolosha, Afflalo, and a few others who are still getting their feet wet, but could turn into great defenders.
(and I am personally leaving Rondo out of the discussion, because he has been discussed ad nauseum on this site...)