There are lots of good plugins but the below four plugins can be blindly installed for any type of site and will add lots of value to your hardwork.
Akismet – The solution to all your Spam problems. Provided by WordPress developers this is default plugin available with installation of wordpress. But you need to supply Akismet key to activate the plugin.
The Akismet key can be obtained from http://wordpress.com/api-keys/. This plugin has a very high accuracy in capturing spam comments and it uses webservices to evolve everyday. If your Akismet misses a spam and you mark it spam, you are doing a great service to the WordPress community by adding to the collective knowledge of the community.
All in One SEO Pack – Best available Search Engine Optimization tool for blogs/newswebsites which run on wordpress. It gives the option of setting various titles, description and meta tags for your site and also inidividual pages and posts. This definitely increases the visibility of websites among search engines.
Google XML Sitemaps – Generates XML sitemaps which is needed for submitting your site url automatically to search engines. This plugin generates sitemaps and allows suitable customization like no of posts to be added in the sitemaps, priority of posts,categories and Home page and all. It also submits content to Google,Yahoo,Ask and MSN.It helps your content in reaching the search engines quickly.
WordPress Automatic Upgrade – A very useful plugin which saves a lot of time by allowing users to upgarde the worpress automatically without bothering to download files or replace etc. A very useful plugin which was launched just one year back. But has hit the popularity roofs very soon. This plugin was developed based on requests from many users.