I had to select Rimington for this one - and not based on the fact he's a Husker. Rather, it was because he set the gold standard for his position - actually for any line position. While Sims was remarkable, I don't think you can say he is the gold standard for running backs.
One problem is that a single skill position player can appear to dominate simply because he has the ball in his hands and the opportunity to change the course of a game on a single play. Conversely, linemen are usually not even noticed - unless they screw up. To be the guy that epitomizes his position - that's enough to swing it Rimington's way, in my mind.