I'm from Detroit... and have been and will always be a Lions' fan, so the answer for me has to be BARRY!!!
The Lions were a pretty mediocre team while he was there (they made the play-offs a few times, but always got beat in the first round), but the same thing was true for Walter Payton and the Bears. Barry's greatness came from the fact that he accomplished what he did playing in a "Run and Shoot" style SINGLE BACK offense for most of his career. Barry didn't have a fullback to blow open holes in the line for him. He was pretty much doing it on his own.
Oh... the other great thing about Barry Sanders... humility. If you asked him this question, his answer would be Jim Brown and then Walter Payton.