I'd really like to be able to say Jim Brown, because that guy was a monster who seems to have had everything you could want in an RB. It just doesn't seem right to make that claim just based on footage, stats, and hearsay. It's about whole games, not just big plays. So, I'm going with Walter Payton. Feel comfortable as I saw Payton play a fair amount and saw Barry play lots, including 10-12 times live.

If RB were just a matter of playmaking/freestyle running, then Barry wins hands down. But an RB has to be able to make short yardage as well as big plays. An RB has to catch passes out of the backfield. An RB has to block. That's all part of the job description.

Short yardage: You just don't want to give Barry the ball on 3rd and 2, unless it's just for kicks to see what might happen (didn't stop the Lions from doing it all the time). Payton could run reliably between the tackles as well as bust it outside.
Blocking: Barry deserves a lot of credit for effort, but he was just an average to good blocker. Payton more solid there.
Catching passes: Barry was above average - even good - catching the ball, but Payton was very good to awesome in that category.

Of course, Barry would have been more productive if he had a better line. But, as noted above, his line wasn't awful, just mediocre. It's not fair to say that the Lions didn't have a passing game though, since the Run and Shoot was definitely a dynamic and dangerous passing game. The Lions never had great QB's, but they did have some awesome receivers. In fact, that wide-open offense (that gave him so much space past the front line) probably accentuated Barry's greatest strengths - his vision, cuts, brilliance in space, and sheer playmaking ability - as well as any offense could. Speculation about stats aside, it would have kinda sucked to see Barry in place of Emmitt Smith in the Cowboys' offense.