Here is a list of top best and must have 15 plugins for wordpress. This list is not exhaustive but every wordpress blog must have atleast these plugins. I have been using these plugins on this site and I am much satisfied with the performance of every plugin listed here.
All in one SEO Pack
For proper Search Engine Optimization(SEO). This plugin is the best plugin that I have found so far for SEO.
Google XML sitemaps
A sitemap helps search engines to properly crawl your site. In short you can say a sitemap is a list all pages that your site. Having a sitemap speeds up the process of getting your site indexed fast.
Block spam comment. This plugins comes pre-installed with wordpress but a must have plugin.
Feedburner Feedsmith Extended
Easily redirect your feed to feedburner with this plugin.
Google Analytics for wordpress
This plugin automatically adds Google Analytics code to your website. There are many more such plugins but I prefer this because it have some advance options that other plugins are lacking. So a must have on my list.
Limit Login Attempts
As the name goes, this plugin limits the number of login attempts. A very important wordpress security plugin. After the set number of allowed login attempts fails, it block the user from trying more passwords for a set time period. It stops brute force attack on your site.
W3 Total Cache
This is a cache plugin. WordPress do not have stored pages. It creates the pages dynamically when requested. This process slows down your site drastically. What this plugin basically does is it makes static HTML pages of your dynamic WordPress site. So when a page is requested multiple times, it is just the cache or the static saved pages is sent. This speeds up your site load time and is very low on your hosting server resources.
This is the best plugin for easy Backup, Restore, Schedule backup and Repair of your database.
Yet Another Related Post Plugin
This plugins displays a list of related post for your visitors to read.
Tweet, Like, Google +1 and Share
This is just a simple social sharing plugin with just 5 social sharing buttons ( Twitter, Facebook Like, Google +1, Linkedin and Facebook Share). It also has a huge list of options to easily customize your site.
Scrolling Twitter Like Google +1 Linkedin and Stumbleupon
A very sleek scrolling social sharing sidebar. The Sharing buttons remains hidden and do not cluster on your page. The icons gets displayed when you take the mouse over the Sharing Bar
Easily monetize your site with adsense. This plugin helps you to quickly and easily add adsense ad units to your blog.
Disqus Comment System
I somehow do not like the default commenting system of WordPress. The only reason for not liking is that even though Akismet is installed and activated, we get a huge lot of spam comments. With Disqus I get none.
Contact Form 7
If you want to set up a contact page where visitors can send some personal message to you. Then this is the best plugin for the job. With this we can easily create contact forms for our visitors.
This plugin automatically redirect to the content the user was most likely after, or show suggestions, instead of showing an unhelpful 404 error.