i´m trying to obtain the max value, but i need to have a reference
ex:

client value
123 1560
123 2000
123 8520
156 1564
154 4666

I need to group client 123 in order to obtain is max value - 8520

Can someone please help me (it´s a list of 40000 clients, and i don´t want
to do manualy)