Walter Payton is my personal favorite. Who knows how good he could have been if he would have played on some decent teams earlier in his career.

However some others that might be metioned are:

Jim Brown - Excellent choice, can't go wrong with a Brown ;)

and

Barry Sanders - Best moves ever, also played on sorry teams.

One thing that works against Brown and Sanders is that they retired early, instead of stretching their careers out and maximizing their rushing yardage.

As far as a running back who flashed brilliantly for a short time.....no one burned brighter for shorter than Earl Campbell.....he was a horse....

In my opinion...
1. Jim Brown
2. Walter Payton
3. Barry Sanders

Jim Brown - Great combination of speed and power

Earl Campbell - Pure straight ahead power

Emmitt Smith - Durable, great runner and receiver

Walter Payton - Another great runner and receiver

O.J. Simpson - He was a great running back on a weak Buffalo Bills team for many years. Getting over 2000 yards in a 14 game regular season is quite an achievement.