I want to say Suh as well because he does have 8 sacks and he's done exceptionally well for a rookie at the DT position. He is a major reason why the Lions are as good as they are despite their horrid record. BUT (and there is a but), I am going to go with Devin McCourtney. A lot of people look at Suh and how he has fared and yes he's done very well considering the team he is on. But I think there is something to be said when you are a rookie, and you are 3rd in the NFL in interceptions. He's actually 3rd to his team mate Asaunte Samuel, and we know that Samuel is no slouch at the corner position. He's a veteran who knows how to defend passes, and I think McCourtey is learning from one of the best. Its not accident that he has 6 interceptions. If anyone is special, I believe its McCourtey. Some may disagree with my answer, but I don't think McCourtey should be overlooked. He's in his 1st year and he's already surpassing those like DeAngelo Hall who is a decent corner and Troy Polomalu. Point being? McCourtey is having a heck of a season and I think that he will get ROY for defense.
Edit: Ok that was a mistake. I completely forgot that Asaunte Samuel went to the Eagles. Human error. Although many of you disagree, I will stand with my answer. I just think McCourtey is not recognized nearly as much as Suh and I don't think its necessarily robbery if he gets the nod for Defensive ROY.