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.