I'll give credit to Bo as being one of the best running backs I ever saw, but he's not the best. There are two guys who stick out to me, one Marcus Dupree was almost 30 pounds heavier and almost as fast. Dupress ran a LAGIT 4.4 40 yard dash. Dupree was at full speed in 3 steps. At OU in 1982 the guy averaged 8.5 yards a carry. Bo and Hershell were never in that zip code. The other is Barry Sanders, from a purely skill point of view he stands alone.
I saw Dupree play on TV several times and every single time he touched the ball it was just one shoe string from a touchdown. If you're linebackers didn't get a good handle on him it was six. DB's had no chance what so ever of getting that guy down, most couldn't even catch him but even if they did he outweighed the biggest safeties by 50 pounds. He was a true north-south runner too. No dancing around, he was coming through the hole weather it was there or not. I hate that his career ended so early.