Depends - they are all rather different tools.
Mac - good for specifics and generally considered the market leader for graphics, CGI and general media work. Of course they are strong machines in their own right, but the sofware range for them is more limited - though there are windows emulators out there to use if one so desires. The bonus of all the components being inhouse made does give the advantage that all the bits are fully comptable
Linux/Unix - Free and strong, but its not for easy street. This is the sort of operating system for people who really understand computers and require specific controls as well as not being afriad to work around things and such to get things to work. Its great for the average geek (which is why they like it so much) but not best suited to your average home owner user
PC - good generalist with the widest range of software out there. Of course we all know the limitations of PCs, but they have their strengths too, easy compataibility with most other systems (its widly used), the wider software range and options from free to commercail as well as a nice big collection of games as well
For me its Windows - games ar a hobby of mine and I am used to the interface (at least till they change it......) Plus most of my software experience is in windows. I am not geeky enough to bother with Linux and MAC would require me to spend a lot rebuilding my games collection and fiddling around with emulators and such