How to enable tinyMCE rich text editor for post Comments in WordPress?

In your functions.php, add the following snippet:

add_filter( 'comment_form_defaults', 'tinymce_comment_4265340' );
function tinymce_comment_4265340 ( $args ) {
    ob_start();
    wp_editor( '', 'comment', array('tinymce') );
    $args['comment_field'] = ob_get_clean();
    return $args;
}

It's working on WordPress 3.5 and Twenty Twelve theme.