Retain select value in select box

If I have understood correctly, you could use get_the_ID() and compare the value with get_queried_object_id() to archive this:

$args       = array( 'post_type'=> 'portfolio', 'posts_per_page' =>100, 'offset'=> 0 );
$myposts    = get_posts( $args );
$current_id = get_queried_object_id();
foreach ( $myposts as $post ) {
    setup_postdata( $post );
    printf(
        '<option%s>%s</option>',
        ( $current_id == get_the_ID() ) ? ' selected="selected"' : '',
        get_the_title()
    );
}