Well, I suppose it depends on what exactly you need. If security is the primary concern, I'd opt for OpenBSD, since its out-of-the-box config is, IMO, the most secure of any free OS.

Of course, FreeBSD is excellent as well, and in the hands of a competent admin is every bit as secure (IMO) as OpenBSD. It also seems as though the FreeBSD crew manages to have a more up-to-date (though probably less-thoroughly-tested) repository of software and libraries.

All in all, I would tend to recommend FreeBSD except in certain niche environments.