<div class="menu">
              <ul>
                <li>@Html.ActionLink("Компания", "Index", "Company")</li>
                <li>@Html.ActionLink("Услуги", "Index", "Service")</li>
                <li>@Html.ActionLink("Центр поддержки", "Index", "Tracking")</li>
                <li>@Html.ActionLink("Контакты", "Index", "Contacts")</li>
              </ul>
            </div>

public static string ActivePage(this HtmlHelper helper, string controller, string action)
    {
      string classValue = "";

      string currentController = helper.ViewContext.Controller.ValueProvider.GetValue("controller").RawValue.ToString();
      string currentAction = helper.ViewContext.Controller.ValueProvider.GetValue("action").RawValue.ToString();

      if (currentController == controller && currentAction == action)
      {
        classValue = "selected";
      }

      return classValue;
    }

<div id="nav">
          <ul>
            <li class="@Html.ActivePage("Company", "Index")">@Html.ActionLink("Компания", "Index", "Company")</li>
            <li class="@Html.ActivePage("Service", "Index")">@Html.ActionLink("Услуги", "Index", "Service")</li>
            <li class="@Html.ActivePage("Support", "Index")">@Html.ActionLink("Центр поддержки", "Index", "Support")</li>
            <li class="@Html.ActivePage("Contacts", "Index")">@Html.ActionLink("Контакты", "Index", "Contacts")</li>
          </ul>
        </div>