Shareaholic: my brother-in-law introduced me to this plugin and it has been one of my favorites. I've tried many share widgets, but this one is head and shoulders above the others. It also includes a Recommendations feature after each post which automatically looks at your post and all other posts and recommends similar posts to the reader. This is an awesome plugin and I honestly saw my traffic double after just 2 weeks of installing.
Akismet- don't even wait to finish reading this post before installing this plugin if you have not already. Naturally your blog comments will get cluttered with Spam and Akismet fixes this issue immediately.
Fast Secure Contact Form- I don't know if this is the best, but it works for me. I tried a couple others and this one was just simple and easy to use. It's nothing fancy and that's probably what I like the most about it.
Feedburner Email Widget- this like the Contact form is really just simple for me and simple for the users. I can't confidently say that it's the best option for you, but it does work for me.
I would love to hear form you what some of your favorite Wordpress plugins are. It still blows me away all that is out there on Wordpress and that so much of it is FREE!