Reset select value to default

You can make use of the defaultSelected property of an option element:

Contains the initial value of the selected HTML attribute, indicating whether the option is selected by default or not.

So, the DOM interface already keeps track which option was selected initially.

$("#reset").on("click", function () {
    $('#my_select option').prop('selected', function() {
        return this.defaultSelected;
    });
});

DEMO

This would even work for multi-select elements.

If you don’t want to iterate over all options, but “break” after you found the originally selected one, you can use .each instead:

$('#my_select option').each(function () {
    if (this.defaultSelected) {
        this.selected = true;
        return false;
    }
});

Without jQuery:

var options = document.querySelectorAll('#my_select option');
for (var i = 0, l = options.length; i < l; i++) {
    options[i].selected = options[i].defaultSelected;
}