After working on more then a thousand WordPress web sites I found that these nine plugins are essential for every WordPress setup. All other plugins are needed per site basis and no matter how much those can be useful not every site needs them.
There are many new plugins which aim to take place of the All In One SEO Pack, but they all fail. This plugin take care your onpage seo (do note that you will still need to take care your headings inside the theme files). Default options are in 99% cases good for any site.
Google XML Sitemaps (by Arne Brachhold)
This plugins generate xml sitemap of your site, which you can add to your Google Webmaster Tools. After installing you need to generate XML sitemap from the plugin options for the first time, after that plugin takes care of everything dynamically.
This plugins generate html and only html sitemap in a page you created and inserted shortcode. It is useful to show of site sitemap to your visitors. It complement Arne’s plugin very well.
As soon as your site grows you will need this. With the help of conditional tags you can display your widgets only on certain parts of your site (etc certain posts, category, frontpage or attachment).
This plugin adds navigation numbers to your blog. So the link juices are spread little bit better. Many themes come with this plugin integrated.
Every site needs proper contact form. This plugins makes that task very easy. Generate your form and insert it with the shortcode where you need it.
Plugin with unique “related” detection algorithm. It support custom types posts too and its own template system.
Fasten your seat belts. This plugin caches your whole site and serve your visitor cached version of the same. This reduce loads and speed loading of the web site. It works perfectly with the WP Minify.