Walker was a great football player, but at the same time he played there was someone better: Bo Jackson.
Bo was simply a freak of nature, such a great athlete that he regularly awed all his opponents in whatever sport he played. The only football player I can compare him to is Jim Brown, who, like Jackson, was an all-American in two sports (football and lacrosse) and became the greatest running back in NFL history.
Personally, I'd put Walker high up in the top 10 of RBs I've seen in person, probably right behind Gale Sayers. But he wasn't absolutely the best by a long shot.
And a last remark: I feel the same way about Bo and baseball that I feel about Duane Allman deciding to take that last motorcycle ride. Why did you keep playing football, Bo? You could have been the best baseball player in history.