Portis is a good running back, but still seems to live off of his numbers from the Running Back factory at Denver. He has only averaged 4.1 yards per carry during the three years he has been a Commander (4.1 average last year as well). Average, but not special. Very comparable to Julius Jones (4.0 YPC). Portis has an edge in the Rushing TD's per game started (.6 Rushing TD's per game started by Portis compared to .5 per game for Jones). Slight edge to Portis, but not as big a gap as his reputation would make some people believe.
That would leave Barber and Betts. Barber has averaged 4.4 over the last two years (only two years in the league), while Betts has averaged 4.5. Barber was light years ahead in the rushing TD's per game played (.7 per game for Barber compared to .2 for Betts. Big edge for Barber based on the TD's per game.