It may seem flippant to just say OpenBSD but it's an awesome piece of software. I tend to deploy FreeBSD for historical reasons but I totally respect OpenBSD for it's complete and utter no nonsense approach to everything.

FreeBSD comes pretty locked down but OpenBSD is locked down as tight as a gnats arse. It's pretty safe to say that if your OpenBSD server is compromised it's something _you_ have done rather then something _you_ have NOT done. It's a good starting point.