ASP.NET Core. Approach 1

If your Controller extends ControllerBase or Controller you can use Content(...) method:

[HttpGet]
public ContentResult Index() 
{
    return base.Content("<div>Hello</div>", "text/html");
}

ASP.NET Core. Approach 2

If you choose not to extend from Controller classes, you can create new ContentResult:

[HttpGet]
public ContentResult Index() 
{
    return new ContentResult 
    {
        ContentType = "text/html",
        Content = "<div>Hello World</div>"
    };
}

Legacy ASP.NET MVC Web API

Return string content with media type text/html:

public HttpResponseMessage Get()
{
    var response = new HttpResponseMessage();
    response.Content = new StringContent("<div>Hello World</div>");
    response.Content.Headers.ContentType = new MediaTypeHeaderValue("text/html");
    return response;
}

Starting with AspNetCore 2.0, it's recommended to use ContentResult instead of the Produce attribute in this case. See: https://github.com/aspnet/Mvc/issues/6657#issuecomment-322586885

This doesn't rely on serialization nor on content negotiation.

[HttpGet]
public ContentResult Index() {
    return new ContentResult {
        ContentType = "text/html",
        StatusCode = (int)HttpStatusCode.OK,
        Content = "<html><body>Hello World</body></html>"
    };
}