Before voting, how are you defining "best"? The most physically talented, most accomplished statistically (single-season or career, since not all of the relevant candidates enjoyed college careers of equal duration) or by the degree to which he surpassed the achievements and/or talent level of his peers? That will definitely influence my decision. In terms of pure physical talent, I would rank Herschel and Bo Jackson as the two best running backs in SEC history. However, Jim Brown at Syracuse possessed comparable size and a similar body type 25 years before those two stepped on the field.
Incidentally, a most excellent question.