FreeBSD: Great legacy, clean codebase. Rock solid.
Linux: Beatiful hackjob, but also in constant development. It's basically up to the end-user/vendor or company to clean it up for increased security/stability. If it were my servers BSD would come first.
NetBSD: my pet research project for understanding Unix code, very neat and tidy.
OpenBSD: for the security freaks. I'd use it for firewalls and that sort of stuff.
Gentoo is sweet, a bit over-the-top-overkill because of all the recompiling. Yeah, you can tailor your own system but look at the time cost.
For the other old UNIX beasts, I have limited experience with them. Could learn pretty fast.