Yeah Barry retired pretty much during his prime. Sad but with the lifespan of the average running back in football, it was actually kind of smart. Best of all-time... I really had to fight with myself between Barry Sanders and Walter Payton... so I'll just say both
As for current guys, Adrian Peterson without a doubt in my mind. The day he works on his pass blocking and starts getting more plays called asking him to receive out of the backfield... it's over. Chris Johnson is just a fast runner, while AD can run around you, or run through you.