Wordpress - How to make pages slug have priority over any other taxonomies like custom-post, post or category

You will need to enable verbose page rules, which make sure that all pages are explicitly defined (instead of using a generic rule). But in addition to that, you need to move these page rules above the taxonomy rules. This is a fairly recent change, I believe it was between 3.0 and 3.1.

I explain how to do this in this answer to a very related question. Together, it is this code:

add_action( 'init', 'wpse16902_init' );
function wpse16902_init() {
    $GLOBALS['wp_rewrite']->use_verbose_page_rules = true;
}

add_filter( 'page_rewrite_rules', 'wpse16902_collect_page_rewrite_rules' );
function wpse16902_collect_page_rewrite_rules( $page_rewrite_rules )
{
    $GLOBALS['wpse16902_page_rewrite_rules'] = $page_rewrite_rules;
    return array();
}

add_filter( 'rewrite_rules_array', 'wspe16902_prepend_page_rewrite_rules' );
function wspe16902_prepend_page_rewrite_rules( $rewrite_rules )
{
    return $GLOBALS['wpse16902_page_rewrite_rules'] + $rewrite_rules;
}