I have used, in order of when I used them: Mac OS 7, Mac OS 8, Windows 95, Windows 98, Windows 98 Second Edition (there were a couple of new features, but nothing major), Windows 2000, Windows ME (In school. It is the devil for those of you who live under a rock.), Windows XP, Mac OS X Tiger, Linux Ubuntu 7.04, Windows Vista (School computers), and finally Mac OS X Leopard.
Personally I prefer OS X, but I really don't mind Windows. In fact I have it on my Mac (I need it for school

). I also have a partition of Ubuntu through Parallels Desktop (Virtual machine program) and I don't really care for it. As iamstrange14 said, it gets boring after a while.