You could always use the search function of this forum instead of starting yet another "Best Plugins" thread...
While there are multitude of the ALL AROUND plugins and I think each WP blogger should have a pre-determined and tested set they personally like - I always take approach to identify... What is my target blog is designed to do? What functionality it will need to get it done? Once you answer those 2 questions above - you will not with 100% certainty what plugins will be needed.
Google Sitemap Generator, Optimal Title, Wordpress Related Posts, cforms, WP Super Cache. Those are just a few of the plugins I use with Wordpress. I suggest you check them out!
Did you check out Caffeinated Content plugin. Checkout the squidoo lens http://www.squidoo.com/caffeinated-content-plugin CC can transform your blog with high quality relevant content in minutes. Best tool for all the bloggers IMHO. Check out this thread for best deal on CC. They include SEO plugin, auto-poster, ebay plugin etc free. http://forums.digitalpoint.com/showthread.php?t=1218245&highlight=caffeinated -Dan
for sure some nice suggestions suggested by the others, you could use nextgen if you have a photopost blog and add-to plugin to make your blog more reachable
Platinum SEO is a must - it's much better than All In One SEO ever was. Here's a list of a few more plugins worth looking into.