Without a doubt, Jack Lambert. The intensity, field smarts, and shear nastiness on the field made him stand out everytime I watched the Steelers play. Jack played the game the way you are supposed to play the game. No fear, and at 100+ percent effort. My favorite quote from Lambert was "if you need to be motivated, you shouldn't be playing the game". I've used that quote a few times while playing high school ball. Of course the "quarterbacks should wear dresses" is a classic.
Of course you can't go wrong with any of the players on the list. If they're a Steelers they are a favorite.