If speed would be main deciding factor, I'd go with FreeBSD/Linux. OpenBSD network stack is still pretty much under big kernel lock so multiple cores won't help

If speed would be main deciding factor, I'd go with FreeBSD/Linux. OpenBSD network stack is still pretty much under big kernel lock so multiple cores won't help as much. Though in the 5.8 and -current branch they've made lot of progress in making the network stack MP-safe. See undeadly.org hackathon reports for details.