Get sublebel value corresponding to maximum within a group

Question

I have a dataframe that looks like this(see the same dataframe in picture):

                        transaction_datetime
client_id   store_id    
0                 81    22
1               3166    5
                3709    7
                5365    29
               10440    7

What I want to get is a grouping, where for each client_id I get the store_id where transaction_datetime is maximum. That is: for client_id 0 I get 81(because there is only one store), for client_id 1 I get 5365.

I see some ways to make it work, e.g.

grouping.reset_index().sort_values(by=['client_id', 'transaction_time', 'store_id'], ascending=[True, False, False]).drop_duplicates('client_id')

But it doesn't look nice. Is there a better way?

grouping.reset_index().groupby('client_id').max() yields maximum values for each column separately(which is expected, I suppose)

enter image description here