There are so many candidates that should be considered for this title. With that said I will suggest a very short list of some players in there position based on skill, ability, longevity, and reputation:
Walter Payton should definitely be placed into the conversation amongst the best running backs to suit up. If anything else, his demeanor was nothing short of legendary. Barry Sanders shines in terms of raw ability to avoid tackles. I think I've seen him get hit cleanly only about 3 times in his ten year career.
Lawrence Taylor revolutionized his position. Not many players, past or present can say the same. Mike Singletary is the best middle linebacker to ever play the game, not to take anything away from Ray Lewis.
Jerry Rice is by far the best wide receiver to play the game and every other wide receiver is compared to him now and forever.
It would be very interesting to see what kind of stats Johnny Unitas would have in today's pass-happy, light-up-the-scoreboard NFL. If only we had a time machine...