Well, you can't really say COMPUTER as they all share the same hardware (now that is, y'know, intel macs and all). But I would say all three. Windows for compatibility, Linux for endless hacking (you can change ANYTHING o.0), and Mac because....
well...
OS X is sexy
Mac got the nice UNIX based operating system, so I chose that. But I use PC, due to the cheaper hardware. But when I have more money, I'll buy a nice mac.