This is what's great about the Big10, it's really hard to answer a question like this. The only real answer is all of them!
Depending on the hawks' conference schedule any given year, who are the teams you really would live to see the hawks beat?
Nebraska, Minnesota, Wisconsin, michigan, MSU, northwestern, Penn St, Ohio? I can't stand all of these teams and IMO, they are all rivals. It's what makes the big 10 so great. To give an answer though, I might say Wisconsin