I have used all 3 major BSDs (Free, Open and Net). FreeBSD is ideal for servers due to its performance. OpenBSD is perfect for security appliances and NetBSD is perfect if you have exotic legacy hardware.
This being said, I have also used OpenBSD for about two years as my daily driver on an old second hand laptop, and I really liked it. With a minimum of configuration, installing software was as easy as Debian (just your pkg_add), and configuration is just super easy since the OpenBSD documentation.
It has improved a lot done then: installing security updates (sysupdate) and upgrading (sysupgrade) from one version to the next is amazingly simple. If your hardware is supported, OpenBSD is just a pleasure to use. Its only default is the lack of “advanced” file systems and volume managers.
Slackware.
It. Just. Works.