It depends on what you want to hack. If you are interested in Windows malware research, kernel hacking, obviously, it would be better to do it on a target platform (of course, you can use virtualization, but there are cases when running real system is vital).
In general, though, you can use any OS you are comfortable with. Personally, I use OS X because I'm happy to have all that great unix stuff: shell, command line utils, etc along with very user friendly interface. I like the fact, that I can do real work without being bothered with tuning look-and-feel features, wasting time in efforts to find good program for editing documents/playing moveis and so on. But Apple computers cost a lot. For this money you, definitely, can buy more powerful hardware. 


So, I recommend you not to begin holy-wars, just give a try to different platforms and chose one that fits you better Good luck!