Notes
Akin to:

import numpy as np
import pandas as pd

df = pd.DataFrame([['1990', 'a', 5, 4, 7, 2], ['1991', 'c', 10, 1, 2, 0], ['1992', 'd', 2, 1, 4, 12], ['1993', 'a', 5, 8, 11, 6]], columns=('Date', 'best', 'a', 'b', 'c', 'd'))
df

	Date	best	a	b	c	d
0	1990	a	5	4	7	2
1	1991	c	10	1	2	0
2	1992	d	2	1	4	12
3	1993	a	5	8	11	6

df['value'] = df.lookup(df.index, df['best'])
df

	Date	best	a	b	c	d	value
0	1990	a	5	4	7	2	5
1	1991	c	10	1	2	0	2
2	1992	d	2	1	4	12	12
3	1993	a	5	8	11	6	5