If you were to have asked me this question 20 yrs ago when MN was in thr struggle to get over the Michigan hump, I'd say MI and IA would be tied for 2nd. Now that rivalry isn't as hateful.
2003:
WI,
MI
IA...
then everyone else
Today:
1. Wi
2.IA
3. NE
(Big gap)
MI then everyone else