You'll have to define exactly what you mean by "max" - lexicographic?

If you're happy with the behaviour of string in terms of
IComparable<string>, you can just call Max:

using System;
using System.Linq;

class Program
{
   static void Main(string[] args)
   {
       var words = new[]
       {            
           "123456",
           "234567",
           "345678",
           "D00001",
           "D00002",
           "D00003"
       };
       
       var max = words.Where(w => w[0]=='D')
                      .Max();
       
       Console.WriteLine(max);
   }
}

I'm somewhat surprised that you can't specify an IComparer<string> to
be honest - although it would be easy to write that as an extra
extension method.