Html.DropDownList selected value from a list<string>
List<string> x = new List<string> { "string1", "string2", "string3", " string4" };
            ViewBag.Relacion = new SelectList(x);

 <div class="editor-label">
            @Html.LabelFor(model => model.Relacion)
        </div>
           <div class="editor-field">
            @Html.DropDownList("Relacion")
            @Html.ValidationMessageFor(model => model.Relacion)
        </div>

<script type="text/javascript">
    $(document).ready(function () {

$('select[name=Relacion]').val($('#Relaciontext').val());

}
</script>
 <div class="editor-field" style="display:none" >
               @Html.TextBox("Relaciontext", ViewData["statecode"])

        </div>
   @Html.DropDownList("Relacion")

ViewData["statecode"]="string1";

@Html.DropDownListFor(model => model.Relacion, (IEnumerable<SelectListItem>)ViewData["Relacion"])