categoria_mais_freq.py

import pandas as pd
import numpy as np

raw_data = {'id': [1,2,1,1,3,4], 'categoria': ['A', 'B', 'A', 'B', 'C', np.nan]}
t = pd.DataFrame(raw_data, columns=['id', 'categoria'])

def cat_mais_freq(user):
    value_count_index = user['categoria'].value_counts().index
    if value_count_index.empty:
        return None
    return value_count_index.values[0]
# t

t.groupby('id').apply(cat_mais_freq)