Are there other ways I can return raw html from controller? As opposed to just using viewbag. like below:
public class HomeController : Controller
{
    publ         
        public ActionResult Questionnaire()
{
    return Redirect("~/MedicalHistory.html");
}
                                                                        Simply create a property in your view model of type MvcHtmlString. You won't need to Html.Raw it then either.
What was working for me (ASP.NET Core), was to set return type ContentResult, then wrap the HMTL into it and set the ContentType to "text/html; charset=UTF-8". That is important, because, otherwise it will not be interpreted as HTML and the HTML language would be displayed as text.
Here's the example, part of a Controller class:
/// <summary>
/// Startup message displayed in browser.
/// </summary>
/// <returns>HTML result</returns>
[HttpGet]
public ContentResult Get()
{
    var result = Content("<html><title>DEMO</title><head><h2>Demo started successfully."
      + "<br/>Use <b><a href=\"http://localhost:5000/swagger\">Swagger</a></b>"
      + " to view API.</h2></head><body/></html>");
    result.ContentType = "text/html; charset=UTF-8";
    return result;
}
                                                                        That looks fine, unless you want to pass it as Model string
public class HomeController : Controller
{
    public ActionResult Index()
    {
        string model = "<HTML></HTML>";
        return View(model);
    }
}
@model string
@{
    ViewBag.Title = "Index";
}
@Html.Raw(Model)
                                                                        Give a try to return bootstrap alert message, this worked for me
return Content("<div class='alert alert-success'><a class='close' data-dismiss='alert'>
×</a><strong style='width:12px'>Thanks!</strong> updated successfully</div>");
Note: Don't forget to add bootstrap css and js in your view page
hope helps someone.
In controller you can use MvcHtmlString
public class HomeController : Controller
{
    public ActionResult Index()
    {
        string rawHtml = "<HTML></HTML>";
        ViewBag.EncodedHtml = MvcHtmlString.Create(rawHtml);
        return View();
    }
}
In your View you can simply use that dynamic property which you set in your Controller like below
<div>
        @ViewBag.EncodedHtml
</div>