Here are the Eight Plugins Which a wordpress blog must have,
Akismet: Plugin to block Incoming spam links and comments
All in One SEO Pack: Make your site search engine friendly, SEO pack plugin is used to add Keywords, description to home page, Format titles, META keywords and 404 titles. And also you can set whether your categories / tags needs to be indexed or not.
Google XML Sitemaps: Automatically creates sitemap for your site and notifies about updates of your Blog to search engines.
Contact Form 7: Be in touch with customers and users, you can create form instantly by adding simple code in page or post.
Theme Tester: Choose the right design for your blog, install this plugin and test with different themes design.
Adsense Daemon: Used to add advertisements in the post, with this plugin you can add ads above or below posts.