That would be 1933! We've only won it twice, 1931 and 1933 (co-champs). Every BIG team except the most recent members has won at least four times as many BIG titles as us-- heck, even the University of Chicago has many more BIG titles than us.
Think being a head coach here is a difficult job?
https://en.wikipedia.org/wiki/List_of_Big_Ten_Conference_men's_basketball_regular_season_champions