I've been running many blogs, I have even built many websites for customers using Wordpress.  This includes using Wordpress as a CMS, or as a blog system.  In every case there are a few core features that each site needs, and the plugins I keep going back to deliver these features just simply work. Here's my list:

This one is a no brainer, it's already included in every Wordpress Installation and the plugin works very well.  Akismet has been key to blocking the spam on many sites I've had the pleasure of working on.  It's easy to use.  All you need to do is activate it, and get a key by creating a Wordpress.com account.

I don't know if this plugin is necessarily the number 2 most important, but in the order that I get them installed this is number 2.  Pinging your blog posts is kind of an overused feature that isn't entirely effective anymore.  But it's still available and still can provide some benefits even if just a small amount of marketing.  The best part about this Wordpress plugin is that it ensures that you don't over ping from your blog on accident.  Any time you edit a post, or schedule a future publish date, your blog can be pinging the same posts over and over, or even pinging when the post hasn't published yet.  This plugin makes sure that it doesn't happen, even provides a nice log so you can see it's effect.