using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    private static int HighestPair(IEnumerable<int> list) => list

        .GroupBy(item => item)

        .Where(group => group.Count() >= 2)

        .Select(group => group.Key)

        .DefaultIfEmpty(int.MinValue)

        .Max();

    public static void Main()

    {

        List<int> list1 = new List<int> { 10, 8, 10,  7, 4, 14, 3 };

        List<int> list2 = new List<int> { 10, 8, 10,  7, 4, 13, 3 };

        List<int> list3 = new List<int> { 10, 8, 12, 12, 4, 13, 3 };

​

        var lists = new[]

        {

            list1, list2, list3

        };

        var result = lists.MaxBy(list => HighestPair(list));

​

        Console.WriteLine("List with highest pair:");

        Console.WriteLine(string.Join(", ", result));

​

        // Highest pair is Max instead of MaxBy:

        var highestPair = lists.Max(list => HighestPair(list));

        Console.WriteLine();

        Console.WriteLine("Highest pair:");

        Console.WriteLine(highestPair);

        // Now, let's order the lists

        var ordered = lists

          .OrderByDescending(list => HighestPair(list))

          .ThenByDescending(list => list.Max())

          .ToList();

        Console.WriteLine();

        Console.WriteLine("Lists ordered:");

        Console.WriteLine(string.Join(Environment.NewLine, ordered.Select(list => string.Join(", ", list))));

    }

}