Jim Brown, it's not close
TP Plamb, for representing us old folks. While I agree that it is no contest re Jim Brown (and I'm a Giant fan and remember him running over and sometimes sweeping around Rosie Greer, Jim Katcavage and Sam Huff) he was the best combination of power, speed and durability I have ever seen.
TP also to BballTim's representing a little younger but still vintage crew with Earl and Bo, but my personal favorite for the brief, spectacular career that he had was the aptly named"Kansas Comet", in that he flamed brightly but burned out all too quickly.
Gale Sayers was the smoothest, quickest, shiftiest RB, kickoff and punt returner that there ever was and he could also run with power. Poetry in motion in a game of violence and collision. The absolute best in terms of raw talent hands down.