Gosh...tough one. Lorenzo Neal had a good start when he was a rookie. He had that one big long run for a touchdown and then messed up his ankle the same year. He was a beast. I remember him going against sumo wrestler and punching Mario Bates in a fight. I didn't understand letting him go. He had potential.
Ironhead didn't have a good rookie season. Was considered a bust the first two years, but in his third year, he blew up. He was already a great blocker but he became a very good runner. Always had problems with Mora and was out of shape. But he was a good all around back. People may not remember this but he was a good receivers. Remember a lot of media phrased him for having soft hands. In my opinion, he was THE BUS before Jerome Bettis.
So I will have to pick Ironhead because he was the best overall package for a full back if he stay in shape.