Two words: Conditional CAPTCHA
First, I want to point out that I did not say THE BEST, just one of the best plugins for WordPress based blogs.
If you're a blogger who uses the WordPress application on a hosted domain, you, like any other blogger know how much work goes into keeping an eye on things in the background. And there are days that the computer demons have other ideas on what you're going to do with your blogs.
One of the primary plugins most may use is called Akismet. Akismet protects your website from slimy spam comments day and night with a pretty decent track record.
Once you've installed Akismet, every time you come back to your blog and log in, the plugin will tell you that you have 'X' number of comments that it has sloughed off into the spam queue for you to review. HA! I don't have the time to go through all the spam every time I log in. On good days, I seem to get a few hundred spam. On bad days, it's pretty annoying.
But each time I'd log in, I'd clear my filter of collected bul... I mean spam. That is until recently when I stumbled upon (No, not via that site), Conditional CAPTCHA. Oh what a god-send! And in god, I must trust when using it.
First, CC needs Akismet already installed. Then when something is treated as if it's spam, CC steps in and presents a screen of text asking the "contributor" to answer a simple question. If they can't, CC handles the spam as I've instructed it to. In case of my websites, if it fails their test, it deletes it. I never see it. Ever. If someone passes the test, it stashes it for me to approve. (Remember, Akismet first marked it as spam).
And I've never had to look at any of that crap spam again. And in god I trust because I hope no one is getting accidentally marked and trashed as spam. (I presume if someone is willing, they could always look at my contact page to let me know they're having troubles.
If you run a WordPress.org based blog, you might want to a peak at the plugin and see what you think. I've been using it for a few weeks and it's already tagged 10,000 spam comments. Hoo ya!