I once read "if meta tags were children they'd be in care after all the abuse they've suffered".
Their value is limited. One of the most important things for search engines is the title. try to put in the title words you think people will use for searching.
However remember people will try anything to get better rankings so huge lists of words (especially if hidden) will be penalised by engines. title and content are mainly what matter. Most search engines criteria for ranking are strictly gaurded secrets (to prevent abuse), so there is no sure fire way to get number one.