I went with middle linebacker because, more often than not, that is the captain of your defense. Most defenses use either their MLB or a safety as the guy who is mic'ed for their DC.
As for Willis, the dude can do it all. Cover tight ends and running backs, stop the run and he's worked on his pass-rushing skills. He quite simply can do just about everything, and neutralize match ups that are head aches for most teams.