Controller:
public ActionResult Filter() { ViewBag.Accounts = BusinessLayer.AccountManager.Instance.getUserAccounts(HttpContext.User.Identity.Name);
hope it will work
@Html.DropDownList("accountid", (IEnumerable<SelectListItem>)ViewBag.Accounts, String.Empty, new { @class ="extra-class" })
Here String.Empty will be the empty as a default selector.
String.Empty