using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var mylist = new List<string>(){

           "itemA.config",

           "itemA.en-us.config",

           "itemB.config",

           "itemC.config", 

           "itemC.en-us.config",

           "itemC.fa-ir.config"

        };

​

        mylist = mylist 

            .Select(s => (Name:s, Culture:GetConfigCulture(s, out string[] tokens), Tokens:tokens))

            .GroupBy(x => x.Tokens.First(), StringComparer.InvariantCultureIgnoreCase)

            .Select(g => g.OrderBy(x => GetOrder(x.Culture)).First().Name)

            .ToList();

​

            string GetConfigCulture(string name, out string[] tokens)

            {

                tokens = name.Split('.');

                if(tokens.Length < 3) return null;

                return tokens[1];

            }

​

            int GetOrder(string culture)

            {

                if(StringComparer.InvariantCultureIgnoreCase.Equals(culture, "fa-ir")) return 0;

                if(StringComparer.InvariantCultureIgnoreCase.Equals(culture, null)) return 1;

                return 2;

            }

​

​

            mylist.Dump();

    }

}