This. There is no one right answer. We have a lot of medium rivalries, and they come and go as programs change and as big matchups happen. I wasn't around when the Bruce Pearl thing happened or during the Bobby Knight era. Therefore, that is not going to weigh as heavily for me as it will for others when you are comparing it to, for example, the Eric Gordon saga, or the previous decade of Wisconsin kicking our butts, or the Luka Garza Iowa teams, or the Zach Edey Purdue teams, or Missouri owning us during the Ayo/Kofi era. There is a legitimate argument for any order of Northwestern, Indiana, Purdue, Wisconsin, Iowa and Missouri. Those are our rivals (maybe Wisconsin less so). The animosity you feel towards those programs is subjective and is different for each person.
It makes me think about watching the Blackhawks during the Toews-Kane era. Who was their #1 rival. It changed almost every year. It was the Red Wings. Then the Canucks. Then the Kings. Then the Predators. Then the Blues. Unless you are UM-OSU, UNC-Duke, etc. rivalries are just not a simple static thing.