Warren Sapp or Ed Reed. Both are among the best to ever play their position, and either would bring an infectious attitude to the defense.
If I could pick two, the second player would probably be Bryant McKinnie. He would definitely shore up the left side of the OL.
I am surprised no one said Michael Irvin, although maybe I missed a post or two. Like Sapp or Reed, he would absolutely bring an attitude, but to the offensive side of the ball.
My comedy pick would be Bernie Kosar, because I would love to see him run RPOs.