So by 'talented' I assume you mean physical ability. I feel that bigger backs tend to get shorted in these types of discussions, but what they do is just as important. Bettis, for instance, had a freakish combination of size/agility/lateral quickness which made him an amazing short yardage running back.
Rather than combining every physical category and trying to 'average' them or something, it might be more interesting to look at who is the best in certain categories or combinations of categories. For instance, Bo probably had the best combination of size/power/speed, Barry had the best agility/balance/escapability, Tomlinson had the best route running/hands, etc.
So what would reasonable physical categories be? Speed (straight line), agility (change direction), acceleration (stop/start), power (running guys over/breaking arm tackles), escapability/balance (tackle breaking/avoiding tackles), hands (receiving), vision (finding holes). Others you can think of?