How to return raw string with ApiController?

You could have your Web Api action return an HttpResponseMessage for which you have full control over the Content. In your case you might use a StringContent and specify the correct content type:

public HttpResponseMessage Get()
{
    return new HttpResponseMessage()
    {
        Content = new StringContent(
            "<strong>test</strong>", 
            Encoding.UTF8, 
            "text/html"
        )
    };
}

or

public IHttpActionResult Get()
{
    return base.ResponseMessage(new HttpResponseMessage()
    {
        Content = new StringContent(
            "<strong>test</strong>", 
            Encoding.UTF8, 
            "text/html"
        )
    });
}