List<SelectListItem> selectList = new List<SelectListItem>()
{
new SelectListItem {Text="text-1", Value="value-1" },
new SelectListItem {Text="text-2", Value="value-2" },
new SelectListItem {Text="text-3", Value="value-3" },
};
//預設選擇哪一筆
selectList.Where(q => q.Value == "value-2").First().Selected = true;
ViewBag.SelectList = selectList;