This is just my opinion, but this Iowa fan believes that ISU is the biggest rival, at least in football. However, it is not the game that means the most, or the one that is the most important. As an example, I wouldn't trade one conference win from 2002 for a win over the clones.
Another way to put this is that the ISU game is the one I least want them to lose. A loss to ISU will be replayed in the state for a year. A loss to Nebraska or Wisconsin, meh, not so much. They are more important games, but don't have the pain associated with a loss to ISU.