Whichever OS doesn't get in your way.

If you have to spend your time patching it, rebuilding the kernel whenever you install something, retuning the security, or otherwise maintain it, I view that as a problem. I don't even like patching my MacOS X box once a month, as I have to reboot it, which is really messing up my uptime.

And well, if you have to learn a whole new OS, that's probably going to be an inconvenience, too.