I have to agree with Acid, besides are you talking about best OS as a server or best OS for on the desktop, because sorry to say it but Linux and some other opensource systems are not better (yet) on the desktop. For example, Linux with a matrox dual head doesn't support 3d, OpenBSD doesn't support smpt nor Java,
but if you are talking about as a webserver/ftp server then the open source systems and also MacOS X beat Windows.