I'd choose an server OS I was familiar with, to start. Something you know is easier to work with then something you don't. Then, keep it up to date, and run onl

I'd choose an server OS I was familiar with, to start. Something you know is easier to work with then something you don't. Then, keep it up to date, and run only what you need to -- shut off things you aren't using. Can you run one of the esoteric OSes mentioned here, with a default stance that is "more secure"? Sure. But, when something breaks, can you fix it? Will you be able to maintain it? Will others? If you are building a security _appliance_, like a firewall, I would say 100% run a default secure OS. If you are running a server, pick something you are good at maintaining, and maintain it. It's not perfect, but if you maintain it you'll do pretty well.