I have OpenBSD running on a Pentium 166 working as a firewall, SMTP and IMAP server, HTTP and HTTPS server, NFS and Samba file server, and, I'm sure, some other stuff. The file serving is slow due to a slow disk drive, but otherwise it's fine.

OpenBSD is nice with regards to security, but one thing to consider is that they got in a huff about licensing issues with IPFilter, so it's no longer the default firewall software. They've developed their own, and I have very little information on it. However, you can still install IPFilter after the fact. IPFilter is what's used under FreeBSD and NetBSD by default. I'm using it on my OpenBSD install and it works quite well.

FreeBSD is the most polished of them by far. It's also probably the fastest, as it only supports a few processors. OpenBSD also only supports a few, but it was started as a derivative of NetBSD, and although it's not really gotten there yet, portability is one of its goals. Otherwise, once installed and whatnot, I doubt that you're going to find huge differences.