Sucuri is the industry leader in WordPress security. It is one of the best WordPress security plugins on the market. They offer a basic free Sucuri Security plugin which helps you harden WordPress security and scan your website for common threats.
Wordfence is another popular WordPress security plugin. They offer a free version of their plugin which comes complete with a powerful malware scanner, exploit detection, and threat assessment features.
iThemes Security is a WordPress security plugin from the folks behind the popular BackupBuddy plugin. Like all their products, iThemes Security offers a nice clean user interface with tons of options.
All in one WP Security and Firewall Plugin
All in One WordPress Security plugin is a powerful WordPress security auditing, monitoring, and firewall plugin. It enables you to easily apply basic WordPress security best practices on your website.
Anti-Malware Security is another useful WordPress anti-malware and security plugin. The plugin comes with actively maintained definitions which help it find the most common threats.
WPScan is a unique WordPress security plugin because it uses their own manually curated WordPress vulnerability database that is updated daily by dedicated WordPress security specialists and community members.They scan your site for over 21,000 known security vulnerabilities.