Also, perhaps OS X? If there's an OS out there where everything from top to bottom seems engineered in harmony, it has got to be OS X...
OSX has a MacOS user interface bolted on top of NeXTStep, and underneath it all there is a BSD/Mach hybrid kernel.
That may be, but a user's perception when using the OS (even as a developer) is one of relative harmony, nowadays - Apple have worked hard to unify these disparate parts and make them work well together - hell they even managed to minify it successfully to produce iOS...
Maybe MINIX? But they don't tend to stick, for some reason.