I suppose that which is *best* depends largely on what you want to do with the system. If you are heavy into
graphics and want access to all the kewlest (and expensive) multimedia software then Mac OS X is your bag. If you want an OS that can be configured in almost any concievable manner (i.e. Server, Workstation, Hacking box, Coding, Firewall,
etc., etc. ) not too mention on most widely available platforms, then Linux may be your bag.

Note: I should mention that Linux is gaining ground quickly in the movie/3D animation industry both as a rendering platform
and a 3D modeling platform. Maya has been available for Linux since version 3.x ( I think ). Also, if you have the cash, and want a very serious 3D imaging system, then you should probably skip both Mac OS X and Linux (for now ) and head straight for an SGI Fuel system running IRIX 6.5.19/20 imho.