using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        int[][] items = {new[]{1, 2, 2, 3}, new[]{2, 3, 3, 4}, new[]{2, 3, 4, 5, 5, 5}};

        var result = items.SelectMany(x => x.GroupBy(y => y)).GroupBy(x => x.Key).Select(x => x.OrderByDescending(y => y.Count()).First()).SelectMany(x => x);

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

    }

}