Remember, no matter which anti-spam you use, one cannot guarantee zero spam. Try to select the one that gives you the best results on your website after some testing on the live website.
Make sure to either activate the auto-deletion of the old spam comments database or do it manually without fail. Forgetting to do so may overwhelm your database storage in a short period of time.
Akismet - This is one of the most popular anti-spam plugins available for WordPress users. You need an API key for this plugin to work seamlessly. It's free for personal blogs though you need to pay a small amount to make it work for business and professional blogs. I use it for my clients as well as for my own blogs. Once the initial configuration is done, it works on auto-pilot without any intervention. You can also start automated deletion of spam queue at definite intervals. It is available in about a dozen different languages and works on multisite blogs too without any hassle.
Anti-spam - As the name suggests, this plugin kills spam effectively. There's no setup or configuration required for this plugin. Simply install and activate it to get a spam free blog. It automatically detects bots and deters them from leaving junk on the site. The plugin itself is tiny and works in the background without consuming your server resources. You may use this plugin in conjunction with other plugins to add an extra layer of security for combating spam. This excellent and handy spam busting plugin works perfectly on all the modern JavaScript-enabled web browsers.
Anti-spam by CleanTalk - This is an all-in-one spam defender plugin saving your WordPress site from spam coming through different means. Apart from regular comment spam, it also protects you from spam registrations, spam coming through contact forms and trackbacks. It's also effective against spam coming from BuddyPress, bbPress and S2Member registrations. The plugin itself doesn't require any complex setup and works instantly after activation. It also protects you from spam trying to penetrate into your blog through WooCommerce review forms.
WP-SpamShield Anti-Spam - This one also protects your blog from spam coming through all popular channels like comment forms, contact forms and through user registration forms. All trackback spam is also effectively busted through this powerful plugin. This plugin is translation ready and can be used easily on multilingual blogs. It works unobtrusively in the background and also keeps track of the number of spam discarded by it. It also works seamlessly with the multisite installation. This plugin also includes shortcode based spam-free contact form.
WPBruiser - This spam plugin uses smart algorithms to detect various kinds of spam bots and relieve human visitors from pesky image-based captcha system. It also prevents brute force login or posting attempts on different types of WordPress pages. Administrators can also put a limit on the maximum number of characters one can write in the comment field. It can also block spam originating IP addresses on an automated basis. White and blacklisting of IP addresses can also be done on a manual basis. A test mode is also available for internal purposes.
Antispam Bee - Like other simple anti-spam plugins, this one also works out-of-the-box. Simply activate and relax watching it slaying spam submissions trying to enter your blog. Like other powerful plugins, this one also works in the background using very minimum resources of your web server. It preemptively blocks resource hogging junk before it starts eating your server space. I tried this plugin on a heavy traffic site receiving a lot of spam. It performed quite well and eliminated over 99% of spam. This is a no-nonsense flexible anti-spam plugin that just works every time.
Spam Master - This impressive plugin deters both comment spam as well as spam registrations. It uses constantly updated spam blocking lists which are updated in real time. It also provides tons of spam busting statistics to get a clear picture of both legitimate submissions as well as the junk entering your blog. It also works with BuddyPress installations seamlessly. Users can also customize the alert or information messages generated by the plugin. You can also create your own local blocking list by specifying select IP addresses, emails and domain names with ease.
No Spam - This efficient plugin uses honeypot method to catch spambots. It adds a hidden field in the form and checks its status after submission to differentiate between humans and bots. It uses JavaScript for its functioning which is enabled in almost all web browsers nowadays. This plugin is lightweight and eliminates all of the comment spam targeted towards the native WordPress comment system. If you experience low to moderate level of spam submissions, this plugin is ideal for your blog. There's no complex setup required to make this useful anti-spam plugin work.
Spam BLIP - This spam busting plugin aggressively kills spam through both user-defined lists and existing DNS blacklists. This helps in eliminating almost all of the spam preemptively. It also deters trackback and user registration spam which can be enabled anytime. It also includes a handy widget to show spam related options and records. If required, it can also block comments originating from tor exit nodes to prevent anonymous visitors leaving obnoxious comments on your blog. The plugin is extremely fast and works quietly in the background preventing all kind of spam.
Stop Spam Comments - I always love those anti-spam plugins that require zero setup. This one also works without any user intervention and kills spam effectively. It works very efficiently without any adverse effect on your web server. Normal human visitors do not experience any kind of problem while leaving comments on your website. All the blocked spam in kept in a queue which can be inspected later to ensure no legitimate comment gets stuck though this happens rarely. Visitors with JavaScript disabled web browser may see a notice to add a key for posting a comment.
Spam Destroyer - As the name suggests, this powerful plugin slays all spam trying to penetrate your WordPress blog. It allows configuring a time frame (in days) after which all the spam comments older than that time duration gets permanently deleted automatically. This keeps your spam queue manageable and allows you to easily scan it manually if required. It also works perfectly with multisite installation without any issues. To some extent, it also blocks spam user registrations. This plugin's developer is quite responsive and promptly answers users' queries if any.
Growmap Anti Spambot Plugin - This is another very popular anti-spam plugin with huge user base. This clearly indicates how effective it is against combating the menace of never-ending spam. It dynamically adds a checkbox within the comment form that cannot be checked by automated spambots. This eliminates almost all of the spam injected through bots. Its smart spam combating system also deters learning bots by dynamically changing the field names of the forms. I'd highly recommend using this excellent anti-spam plugin on one of your professional blogs.
Squelch Unspam - This plugin uses a common spam busting technique of dynamically changing the name of comment form input fields. It works perfectly even if the JavaScript has been disabled within a web browser. It also adds an invisible honeypot within the form that helps in catching the spam bots. WooCommerce forms are also guarded by this flexible plugin. Even if you're experiencing an overwhelming amount of spam, this plugin works perfectly eliminating all of the junk directed towards your blog. Simply install this plugin to a spam free blog.
BotSmasher - This handy plugin gives you triple protection against spam coming through different sources. It protects your blog against spam coming through comments, user registrations or through regular contact forms. Spam protected contact forms can be easily embedded anywhere on the page through a shortcode. It works quietly in the background keeping your website free from spam. It's available in several popular languages like German and Portuguese. It also allows you to customize the style of spam protected contact forms embedded through shortcodes.
SpamLord - This plugin protects you both from comment spam as well as trackback spam. It has four different modules which eliminate almost every type of spam reaching your website. This unique plugin also includes several interesting features like sending false positive to the bots making them think they've succeeded in their mission. In some situations, you may want to reply a spammer in his own language leaving random sarcastic replies automatically. If someone is trying to submit replies too quickly, this plugin detects it automatically and quickly blocks it.