Reducing id combinations in pandas dataframe

Question

I have a pandas dataframe that has combinations of two id columns such as this:

ID1	ID2
A	B
A	C
A	D
A	E
B	C
B	D
B	E
C	D
C	E
D	E
F	H
I	K
K	J
G	F
G	H
I	J

Here we have the choose 2 combinations for ABCD, FGH, IJK.

I would like to only keep the rows for the value with the most ID1's for a particular set. For ABCD this would be A, for FGH this would be G, and for IJK this would be I. Resulting in the following:

ID1	ID2
A	B
A	C
A	D
A	E
I	K
G	F
G	H
I	J

Answer 1

Calculate the count of unqiue values in ID1, then inside a list comprehension for each set calculate the index of maximum value, finally use these indices to filter the rows in dataframe

c = df['ID1'].value_counts()
i = [c.reindex([*s]).idxmax() for s in ['ABCB', 'FGH', 'IJK']]

df[df['ID1'].isin(i)]

---

   ID1 ID2
0    A   B
1    A   C
2    A   D
3    A   E
11   I   K
13   G   F
14   G   H
15   I   J