Absent context and purpose, the question can't be answered.
Building a storage server? You'll want ZFS and therefore FreeBSD. A gateway router and firewall? Probably OpenBSD + pf (although FreeBSD + pf works fine, there, too).
why would you prefer OpenBSD for server if i may ask?
for what purpose ?
i would prefer FreeBSD for a laptop/desktop/workstation, but openbsd for a lean/mean server...