If I would have a choice to install the only one plugin for my WordPress (how glad I am that I don’t have to make this choice, by the way), I’d go with Spam Karma.
Last week I installed it to see if it was any good. It is. I needed just a couple of days to realize how big of a problem SPAM comments still were. They weren’t appearing automatically on my blog, but I was getting an email every time a new comment was submitted, and I had to mark it as SPAM in the admin interface. Of course, there is a shortcut ‘Mark all as SPAM’, but still, it required an action.
With SPAM Karma, I don’t have to do anything at all anymore. It checks all the comments and automatically marks SPAM as SPAM and aproves the good ones. I don’t get emails about each SPAM comments anymore. Rather a daily digest that tells me how many SPAM comments were caught and where I can review them, if I wish. For each approved comment I still get a notification – so that I could reply faster. And on those rare occasions when SPAM Karma can’t make up it’s mind, it sends me the request for approval.
In short, it works better than very good. It works excellent. And I didn’t even do any configuration what-so-ever (although there are plenty things to tweak). Just intalled it as it was.
With this plugin there is no need to use captchas or limit commenters to logged in only users. Great!
P.S.: I’ve also recommended this plugin to Michael Stepanov and he seems to like it too.