I am looking at it from a subjective point of view. The best defensive players are those who can make plays in all facets of the game: run, coverage & blitzing. The LB position is created to do all these things (much like it's offensive counter-position, RB, which can be used to run, catch & block). While Harrison won the DPOY, he is very limited in his coverage skills, plus he doesn't have the speed of Ware to gain ground on a ball carrier. Polumalu & Reed are the other two players who are the elite of the elite in terms of defense, yet their position (Safety) naturally dictates that they will not make their presence felt in pass rushing on a consistant basis, so that eliminates one of the big 3 facets of defense.
Now, with all that stated,aside from Ware, Harrison, Polumalu & Reed, who do you think is the best defensive player in the league?