@Html.DropDownList("SID", new SelectList(@ViewBag.SiteTypes, "SID", "SiteName"), "-select SiteName-")

List<SelectListItem> yourOptions; //populated however you already populate them

yourOptions.Insert(0, new SelectListItem("--select Site--", 0));
ViewBag.SiteTypes = yourOptions;

@Html.DropDownList("SID", new SelectList(@ViewBag.SiteTypes, "SID", "SiteName"))