They are all good in their own right.
Right now i have a macbook pro with both windows 7 and ubuntu 9.04 as virtual machines.
Mac OSX in my opinion (as someone who is using it) is over-hyped. Sure OSX is a good solid OS but its not nearly as good as apple wants everyone to believe. Also, as a power user and someone who is comfortable working solely in terminal, i feel like the gui on my mac could do alot better at using some of the features that unix offers (i do almost all of my interactions with files from terminal [copy, rename, edit files, etc]). Also, Adobe CS4 for mac is something i use almost every day.
Windows 7 looks to be great and, as always, there is an app for anything you want to do. Some people complain about how windows gets viruses but nowadays, you need to be pretty stupid to get a virus as hardly any can be gotten without you clicking on them.
My favorite OS to be on though is ubuntu. IMHO, that is the best Linux distro out there and I highly recommend duel booting it with any os. The only downside is that you will have to rediscover what programs to use and for certain tasks there may not be a program.
Summary: Windows for apps, mac for design, and ubuntu anytime i can.