Personally, I prefer Mac's OS X. I used to use Windows XP. I liked it well enough, but I had problems when after a year or two of working fine, it decided stop booting. Eventually I fixed it by reinstalling right over the old system files, but it was still a pain. I also had make sure I had all non-replaceable files backed up elsewhere, in case it erased some while reinstalling XP. Even before XP I was a Ubuntu (Linux) user. While I support and love open-source programs, the biggest problem I had with Ubuntu was program compatibility. Many of the programs I wanted to run would not run on Ubuntu, and I couldn't find a good driver for my graphics card under Ubuntu either. These were some of the issues that eventually drove me to XP. My latest switch has been to Mac. Frankly, I like it best out of any OS I've ever used. The UI is much simpler, and it doesn't have some of the problems that Windows has. Windows is like a generic T-shirt. While it can be worn on nearly anything, it doesn't fit perfectly. Mac OS X is like a tailored suit jacket; it only fits a few, but it fits perfectly. Linux is almost like a patch jacket, being continuously added to, with a rather awkward fit sometimes. Does it make any sense? The other thing I like about Macs are there compatabily with other operating systems. For Windows only programs, I can just use boot camp, allowing me to start up Windows or OS X on my computer, depending on my needs. Finally, while I have never run Vista on my computer, I've had extensive experience with it. It has a massive number of bugs and problems, and I avoid using it whenever possible. From what I've seen, Windows 7 looks okay though.
My final verdict on my operating system of choice: Mac OS X