I voted for Iowa, though it was a tough decision for me between that game and Michigan State considering we were beating them for most of the game in East Lansing last season, boy was that a fun game to watch, just watching the Gophers hold their own against a top 15 team was so cool.
That said, I have heard Nebraska fans say that every year the Huskers inexplicably blow one at home every year against a team they should beat. That certainly holds true for the last three years.
2009 - Iowa State
2010 - Texas (Texas went 5-7 that year and Nebraska was ranked #5 at the time)
2011 - Northwestern
2012 - Minnesota?