Like some of the others have said- this is a tough one. But, I'd have to got with DJ. Hali has always been good, and his stats this year were an indication that there were more weapons around him- freeing him up to go get the QB!
But DJ, getting a chance to be on the field a LOT more, stepped up, and played incredibly well to solidify our middle D.
Cassel was much improved, Bowe was still about the same, and MANY others continue to get better, but I think the most marked improvement was DJ's.