ramco1917 says:
using (var yt = new YouTubeService(new BaseClientService.Initializer()
{
    ApiKey = "APiKey",
}))
{
    StringBuilder htmlTable = new StringBuilder();
 
    var request = yt.Search.List("snippet");
    request.Order = "MostViewdVideos";
 Replace above with below code.
using (var yt = new YouTubeService(new BaseClientService.Initializer()
{
    ApiKey = "APiKey",
}))
{
    StringBuilder htmlTable = new StringBuilder();
    var request = yt.Search.List("snippet");
    request.Order = Google.Apis.YouTube.v3.SearchResource.ListRequest.OrderEnum.ViewCount;
}