Wordfence is the best plugin. It protects your WP site against hacking and informs you whenever you have to update something..
Yoast allows you to add robots noindex meta tags for specific types of pages and to build an xml sitemap. It also allows you to edit titles and description meta tags. The SEO aspect is overrated though and leads people to spam. You get most of the same features with All in One SEO Pack plugin which I find easier to use.