Here are a few of my favs:
-Yoast SEO. By far my favorite plugin for SEO
-Google News Sitemap. Automatically submits all of your content to google to be indexed in google news
-Email Notify. Allows you to collect e-mail addresses from comments, and also allows users to subscribe to future comments so they are notified if they get a reply to their comment
-Sharebar. Excellent plugin to place your social media buttons in the best place
-WP Page Navi. Allows you to have more control over your pagination, rather than just having older entries. Helps google to crawl your older pages of content more easily.
-Akismet. Necessary to block out comments!
-Better WordPress Minify. Helps you to minify your CSS and JS files for a faster site load time.