Combo Box by default selected the last option

<select>
    <option>item1</option>
    <option>item2</option>
    <option>item3</option>
    <option>item4</option>
    <option selected="selected">item5</option>
</select>

A plain JavaScript solution:

select.selectedIndex = select.options.length-1;

Demo

Do something like this:

$(function() {
    $("select option:last").attr("selected", "selected");
});