Code is fine, just needed to use OrderByDescending to find the best candidate, however you are missing out one validation which is why you only receive 60%. If you complete the last validation you should get 100%.

Best,

using System;
using System.Linq;
using System.Collections.Generic;

namespace Exam
{
    class Program
    {
        static void Main(string[] args)
        {
            var listOfContests = new Dictionary<string, string>();
            var bestCandidate = new Dictionary<string, Dictionary<string, int>>();
            var listResult = new Dictionary<string, int>();

            while (true)
            {
                var input = Console.ReadLine().Split(':').ToArray();

                if (input[0] == "end of contests")
                {
                    break;
                }

                var contest = input[0];
                var passwordForContest = input[1];

                listOfContests.Add(contest, passwordForContest);
            }

            while (true)
            {
                var input = Console.ReadLine().Split("=>").ToArray();

                if (input[0] == "end of submissions")
                {
                    break;
                }

                var contest = input[0];
                var password = input[1];
                var user = input[2];
                var points = int.Parse(input[3]);

                if (listOfContests.ContainsKey(contest))
                {
                    if (listOfContests[contest].Contains(password))
                    {
                        if (!bestCandidate.ContainsKey(user))
                        {
                            bestCandidate.Add(user, new Dictionary<string, int>());
                            listResult.Add(user, points);
                            bestCandidate[user].Add(contest, points);
                        }
                        else if (bestCandidate.ContainsKey(user))
                        {
                            // MISSING VALIDATION
                            //Save the user with the contest they take part in
                            //(a user can take part in many contests) and the points
                            //the user has in the given contest.If you receive the same
                            //contest and the same user update the points only if the new ones are
                            //more than the older ones.
                        }
                        else if (!bestCandidate[user].ContainsKey(contest))
                        {
                            bestCandidate[user].Add(contest, points);
                            listResult[user] += points;
                        }
                    }
                }
            }

            foreach (var (key, value) in listResult.OrderByDescending(x => x.Value).Take(1))
            {
                Console.WriteLine($"Best candidate is {key} with total {value} points.");
            }

            Console.WriteLine("Ranking:");

            foreach (var (key, value) in bestCandidate.OrderBy(x => x.Key))
            {
                Console.WriteLine(key);

                foreach (var (contest, points) in value.OrderByDescending(x => x.Value))
                {
                    Console.WriteLine($"#  {contest} -> {points}");
                }
            }
        }
    }
}