These are the 5 plugins that are essential to all my WordPress ran websites.

All in One SEO Pack

This plugin simply optimizes your WordPress installation for search engines. As you know, WordPress doesn’t include Meta Keywords and Meta Description in its installation, so you need this plugin to add those. It also overrides any title, Meta Keywords and Meta Description per post/page.  All in One SEO Pack generates META tags automatically, provides advanced canonical URLs, support for Custom Post Types, automatically optimizes the titles and so on.

http://wordpress.org/extend/plugins/all-in-one-seo-pack/

 

Simple Google Sitemap XML

This plugin generates an XML sitemap which can be used with Google’s Webmaster Tools. A new sitemap automatically gets created every time you publish or deletes a new post/page. You can also configure what to store in your XML file.

http://wordpress.org/extend/plugins/simple-google-sitemap-xml/

 

Redirection

This manages 301 redirection without accessing your .htaccess file, this is important if you are migrating pages from an old website, it also automatically adds a 301 redirect when a post URL changes. This plugin also monitors 404 errors in your website.

http://wordpress.org/extend/plugins/redirection/

 

Contact Form 7

This plugin can manage multiple contact forms in your WordPress installation, you can fully customize the form and mail contents easily.

http://wordpress.org/extend/plugins/contact-form-7/

 

Akismet

This plugin checks the comments in your WordPress installation against the Akismet web service to see if they are spam or not. It also lets you review the spam it catches.

* included in the WordPress installation