I think it depends on your purpose. OpenBSD is focused on correctness, security ("secure by default"), portability, and good documentation. (it is my choice.)

I think it depends on your purpose. OpenBSD is focused on correctness, security ("secure by default"), portability, and good documentation. (it is my choice.) It is the only one I know of that has had only 2 of the worst kind of security hole (remote hole in default install) since about 1996. It is the source of pf (firewall) and OpenSSH and I think other things that are used elsewhere. OpenSSH is widely trusted. It is not as fast at disk i/o as FreeBSD and doesn't have ZFS or Bluetooth support, or USB support for some use cases, I gather (though I use it with some USB things all the time). FreeBSD seems focused on performance and stability, and some people say it is not as secure as openbsd, especially out of the box. Other people debate, and on it goes. It has the most users of the 3. NetBSD seems focused on portability, and ... I don't know, fun hobby-ness? I know least there.