I know Michigan hasn't played Notre Dame in a minute and MSU is on the schedule every year. But for me (and I would imagine many Michigan fans) rank the Wolverines Rivals as follows:
1. Ohio State
2. Notre Dame
3. Michigan State
The Michigan - MSU thing is more of a nasty distraction then a rivalry. MSU fans know that and they hate that. When ND was on the schedule every year, there would be two must wins for me: ND and OSU. Some years it was the first game and the last game of the season.