The question is very generalized. You need to be very specific on the type of Security you are looking for in your latest install. There are many Security Plugins available but each having its own use and purpose.
Cross-Site Scripting (XSS)
Remote file inclusion
Several DoS conditions
And many more, including all of the OWASP Top Ten security vulnerabilities.
For Spam Protection you can use AKISMET but for overall site security, you can use [[B]6 Scan Security[/B] and [URL="http://wordpress.org/extend/plugins/better-wp-security/"][B]Better WP Security[/B]](http://wordpress.org/extend/plugins/6scan-protection/) together.