get last alphabetic value without using lists

I have some strings, and I would like to get the letter that is last in alphabetic order.
I know how to do it on a List (ordering the list), but what I would like is such a function :

private string getLastValue(string a, string b)
{
   //????
}

getLastValue("a","b") may return "b"

getLastValue("azerty","qwerty") may return "qwerty"

>Solution :

You can just compare strings with a help of StringComparer, e.g.

private string getLastValue(string a, string b) =>
    StringComparer.Ordinal.Compare(a, b) > 0 ? a : b;

Note, that you can choose the comparer required: Ordinal, OrdinalIgnoreCase etc.