values  ii
0     3.0   4
1     0.0   1
2     3.0   8
3     2.0   5
4     2.0   1
5     3.0   5
6     2.0   4
7     1.0   8
8     0.0   5
9     1.0   1

bii2=df.groupby(['ii'])['values'].agg(pd.Series.mode)

bii3=df.groupby('ii')['values'].agg(lambda x: pd.Series.mode(x)[0])