OSX server is pretty good. it does run on a Linux kernal, however.
To clear up any misconceptions: Mac OS X (both client and server) are derived from BSD UNIX, not Linux. It runs on a microkernel called "XNU", which combines the Mach microkernel with the FreeBSD kernel with "I/O Kit", a C++ interface for drivers.
Add all this to the usual GNU tools, and you have a solid UNIX implementation. Just like adding the GNU tools to the Linux core OS makes that a solid UNIX implementation as well.