That is what constitutes a rivalry to you? By using your logic the Iowa/PSU game is just as much a rivalry, if not more so since Iowa has an even better record against them than NW has against Iowa lately.
Might as well add tOSU as well. They have owned Iowa a lot more than NW has. So Your list would actually look something like ISU, Wisc, tOSU, PSU, NW, Minn. Gotcha.
I classify a rivalry as a game that I circle on the calendar on a consistent basis, or a trophy game. Over the past 5-7 years, that would be ISU, Northwestern, Wisconsin, and Minnesota.