I've used for an extended period of time Mac OSX (Tiger and now Leopard), Windows (3.1, 95, 98, ME, 2000 NT, XP, and Vista on a friend's comp), Linux (Ubuntu, Hoary Hedgehog and Breezy Badger).
Windows: Vista is garbage at this point, stay with XP. After using everything else, I've done a clean install of XP, installed BARE minimum utilities, and use it as a pure game machine. It only wins in games for now, as EA and I believe Id have announced future developing for Mac.
Linux: Powerful as hell, but with great power comes great responsibility. It has come leaps and bounds with the apt-get repositories and such for installing software, but it's still not as easy as windows or mac. Configurability is the name of the game, allowing you to extract every ounce of power out of your system and customize your experience to your needs. The open source community is fantastic, providing many great and free alternatives to common software. Still lacking in the game department though, and proprietary drivers can still be wonky (I'm looking at you, ATI)
But, my OS of choice is hands down, Mac OSX. It's a good balance between ease of use, power, and eye-candy. Leopard is certified as fully Unix compliant as well. Installation and uninstallation of programs is easy as dragging an icon, freeware looks and performs great for the most part, and it's feature rich and fairly configurable. I use this for everything other than gaming and I've never looked back since the switch.
