Le'Veon Bell did it all. An outstanding runner who may have better receiving ability than taking it out of the backfield. He's a do-it-all runner, and because of it, he's earned his respect.
Don't get it wrong, Willie Parker did remarkable things. He's engraved himself in Steelers history, but his short-lived NFL spark isn't enough to compete with Bell's start in Pittsburgh.
Leaving the question with a simple answer - Le'Veon Bell is the third-greatest running back in Steelers history. Franco Harris and Jerome Bettis reign one and two. And Le'Veon Bell made his way right behind them.