using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program {

    public static void Main() {

        var lista = new List<string> { "AAA", "BBB", "CCC", "ABB", "ABC", "ACC", "ACD" };

        var padrao = "A";

        foreach (var item in lista.OrderByDescending(x => x.Select((c, i) => x.Substring(i)).Count(sub => sub.StartsWith(padrao)))) {

            Console.WriteLine(item);

        }

        Console.WriteLine();

        padrao = "AB";

        foreach (var item in lista.OrderByDescending(x => x.Select((c, i) => x.Substring(i)).Count(sub => sub.StartsWith(padrao)))) {

            Console.WriteLine(item);

        }

    }

}