If you frequently experience malware infections on your WordPress site, get it audited by a security expert before trying these plugins. They may find a loophole or a backdoor on the server itself.
Although under the hood, almost all of the malware scanning plugins are the same, the difference lies in the interface and the promptness at which the database is updated. Pay attention to both of them.
Sucuri Security - Undoubtedly, it is one of the most popular security plugins available for WordPress. Once installed and activated, it can scan for several common security threats including malware and viruses. It uses several APIs to fetch the latest malware detection data for securing your blog from the latest threats. You can also use and activate several generic security options available through this plugin to harden the overall security of the blog. It can also detect changes made to the WordPress core files that can be exploited by hackers to penetrate your installation.
Anti-Malware - This is an all-in-one solution to keep check on malware threats affecting WordPress blogs. It proactively scans for malicious scripts and helps in cleaning the infection, if any. You can either start manual scanning or can customize the scanning schedule. As an additional security layer, this plugin automatically patches some known scripts with the secure version. The plugin itself is frequently updated and is known to automatically remove some of the most common infections affecting WordPress blog. It's lightweight with user-friendly and useful controls.
Wordfence Security - This is a powerful security solution for WordPress users that not only scans for malware but also implements a firewall on your web server. It also supports two-factor authentication system for adding an extra layer of security to prevent unauthorized access and brute force login attempts. This plugin also has the ability to automatically repair corrupt WordPress core and plugins files. It also provides a real-time view of incoming traffic that includes visits made by bots and spiders. In a nutshell, this plugin is a comprehensive security solution for WordPress.
OSE Firewall™ Security - Although the name suggests that this solution only activates a firewall for a WordPress blog, reality is totally different. Apart from activating a powerful firewall, it also includes malware and virus scanning module to detect malicious code and scripts. It also keeps guard against SQL injection attacks through bots. One can also use its powerful IP blacklisting feature to prevent access from known bad neighborhoods. It works out-of-the-box and requires minimal setup and configuration. This plugin is frequently updated to protect from the latest threats.
AntiVirus - The name itself tells what this plugin is all about. This plugin can automatically scan your blog for malicious scripts on a daily basis and can email the report upon completing the process. It also scans database tables and theme templates for malware and viruses without putting any load on the server resources. Optionally, you can perform manual scans for select parts of your blog installation. One can also whitelist a specific file or directory to prevent false alarms by the plugin. It's lightweight and is readily available in several popular languages.
WPGuards - This plugin is more than just a security plugin. Apart from an automated daily scan of malicious scripts, it also provides a powerful backup solution to safeguard your blog's content. You can also monitor your blog's uptime through this handy solution. Its scanning module is lightweight and can detect most malware that generally affects WordPress sites. It also has several server level diagnostic tools for advanced users to troubleshoot various technical problems associated with the web server of your WordPress installation. I've used it for a couple of client websites.
All In One WP Security & Firewall - This plugin has a very good interface with tons of security features. Apart from regular malware and virus scanning, it also monitors user login activity. It also proactively monitors changes made to the database tables and file system. Its IP blacklisting module supports wildcard characters to deny access to a large range of IP addresses. This plugin also takes backup of critical WordPress files that can be easily restored with a single click. You also get reliable protection against brute force login attempts made by hackers.
SmartFilter Security - This flexible plugin has been specially designed to prevent injection of malware within post content and the associated comments. Its powerful scanning engine instantly filters out malicious code sent by bots and hackers. It can also fix broken themes and templates gone wrong due to malware. Once activated, it quietly works in the background without leaving any visible footprint on the server resources. Blog owners often confuse this plugin with a typical anti-spam plugin. But it is a full-fledged malware detection engine securing different parts of your blog.
Total Security - As the name implies, this plugin proactively checks and prevents infections on your WordPress blog from malicious scripts and code. After activation, it performs a detailed examination of the blog installation for vulnerabilities and loopholes. It can be regarded as a full-fledged security audit of the blog. It also has a WordPress core scanning and repairing module that fixes problems associated with the system files of WordPress. It can also generate detailed 404 error logs to check the generated pattern of these errors for detecting abnormal or suspicious behavior. 
Security Ninja - This is one of the best malware scanning plugins available for WordPress bloggers. It can detect 40+ malicious code and scripts and can clean them completely. With a single click, it can scan and check both WordPress core files as well as database tables for infections. This plugin works seamlessly with multisite installations too in different web server configurations. Administrators can also view the infected file source code quite easily through this plugin. Power users will find this security plugin extremely useful and effective.
6Scan Security - This plugin contain several critical modules that provide a comprehensive security suite for your WordPress blog. Apart from powerful malware and virus scanning engine, it also includes a solution for site backup. It also includes a web analytics module to track the inbound traffic of your blog. Most infections are repaired automatically by this plugin which relieves you from the associated technical process. Its malware scanning process runs in the background without affecting your routine blogging tasks. The interface is slick to easily manage the security of the blog.