https://www.youtube.com/watch?v=zp4BMR88260 Answer

Program.cs

using System;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.IO;
using System.Linq;

namespace ConsoleApp1
{
    static class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(File
                .ReadAllText("words.txt")
                .Split("\n").Length);
            var possibleLetters = "ABCDEFHLNPRSTUY".ToCharArray();
            var words = File
                .ReadAllText("words.txt")
                .Split("\n")
                // Convert every word to all UPPERCASE, easy comparison later
                .Select(s => s.ToUpper())
                .ToImmutableList()
                // Longest word first
                .Sort((a, b) => b.Length - a.Length);
            // First valid entry is longest word (because of the sorting)
            var first = words.First(w => w.ToCharArray().All(possibleLetters.Contains));
            Console.WriteLine(first);
        }
    }
}