using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var text = "<p><h1>Antalya</h1>, Türkiye'nin bir ili ve en kalabalık beşin</p><p><br></p><p><iframe width='560' height='315' src='https://www.youtube.com/embed/T-UEfYsyzuo' frameborder='0' allow='accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture' allowfullscreen=''></iframe>                                        </p>";

        string pattern = @"(<.+?)\s+style\s*=\s*([""']).*?\2(.*?>)"; 

        //Deleting style values pattern

        string substitution = @"$1$3";

        //Deleting style values Substitatin

        Regex regex = new Regex(pattern);

        var removedStyle = regex.Replace(text, substitution);//We are deleting style values

        var patternX = @"(?!</?h1>|</?h2>|</?h4>|</?h5>|</?p>|</?ul>|</?li>|</?b>|</?iframe>|(?:<iframe[^>]*)(?:(?:\/>)|(?:>.*?<\/iframe>)))<.*?>";

        //(Keeped tag list seperated with " | " ) and <.*?> remove all other 

        var resultx = Regex.Replace(removedStyle, patternX, String.Empty, RegexOptions.Multiline);

        Console.WriteLine(resultx);

    }

}