I would recommend FreeBSD. It's easy to administer, and it's very stable. If you look at the highest average uptimes on
netcraft, you'll see how stable both FreeBSD and BSD/OS are.
If you are paranoid about security, I might recommend you OpenBSD.
If you want a system
very easy to administer, I would definitely recommend Mac OS X server.
I am currently building a Apache server on FreeBSD, with Sime's help (for which I am very grateful).