Based on cumulative record of opponents*, ESPN blogger Brian Bennett claims ND's 2010 schedule was the toughest. In comparison, he ranks Michigan's 15th, which is third in the B10 behind Illinois (10th) and PSU (14th). Also interesting, tri-champions MSU, OSU and Wisky come in at 44th, 55th and 70th, respectively.
* I have not gone back to check Bennett's calculations, but it appears he may be including only FBS opponents. If that is true, it might explain why ND comes out on top.