i personally just hate the mac's interface. too much flashy animated bullstuff + that round buttons thing. not really something major but it just annoys me a lot.
windows is pretty cool, i mean, probably most applications are developed for windows, and it's probably one of the easiest most stable operating systems out there. almost never had a problem with it. it's nice, attractive, and has a nice level of customization and personalization.
linux, so far, despite all of the problems i've been having recently, has been my favorite. it's just a completely open development platform that you can do whatever the forget you want to and with it. with linux, you can have it look however the hell you want, quite literally, with no limitations in customization, and it runs a hell of a lot faster too. linux is also surrounded in a nice community of like-minded developer types who i find just the most charming type of people, and even with a million and a half problems, there's a solution to everything.
however i blame most of these problems on my curious self, so that point is irrelevant. with the speed, customization, ease of deployment, heavy programming/scripting development support, and it's price of absolutely nothing, linux takes the cake for me.