My vote goes to XP Pro. Here's why:
I run a dual-boot system between XP Pro and Ubuntu 7. I love Ubuntu. It's easy to use, easy to learn, comes with tons of free software, and puts tons of other free software right at your fingertips. You have more control over the GUI, which provides for more personalization. The screensavers are far cooler than any others out there, and we all know the only point in having a screen saver anymore (since everyone's on LCD monitors now) is to make your computer do something spiffy when nobody's looking. Version 7 has some REALLY great features, too. The graphical enhancements are the least of them. Reading/writing NTFS files and directories makes file transfer between my two partitions easy so I don't have to store my 60 gigs of music on two partitions of the same HDD. A built-in partition table editor (that works faster and cleaner than any partition table editor I've ever seen) lets me readjust my HDD whenever I run out of space here or there.
But I'm a video gamer, so that pretty much throws Linux out the window for me. It's an amazing operating system. It never crashes, it's so easy a four-year-old could install it, it's FREE (a hard bargain to pass up considering the price of software these days), and it comes straight off the disc with immeasurable everyday-use functionality (a point that's already been made, and made well). When it comes to gaming, though, there's not much it can do. WINE doesn't run my games very well.
So that means Windows. (I will kindly ignore the fact that Mac OSX exists, because I've had to work on Apples before, being a Geek Squad member, and after all that, I swear to God I will never own one, not until the price goes down significantly.)
I work on Vista computers all the time. Every time one of our computer salesmen sells a PC and I do the setup, I have to touch Vista. I've seen people return PCs because of Vista. Here are common problems:
First: Very little gaming. Most new games work under Vista pretty well. Some older games work well, too. I personally can vouch for Guild Wars: Prophecies. But if your game is more than a year or two old, forget it. Vista loses compatibility.
Second: Realtors HATE Vista. This is because they use a program called MLS (Multiple Listing System). MLS runs on an Oracle database. Microsoft Internet Explorer 7 has no support for Oracle database systems. This forces realtors to use Firefox or some other thrid party browser. Firefox is, of course, better for them, but it's frustrating to be forced to adapt to something so different from what you're used to, especially when you're having to do it for your business.
Third: The version of Remote Desktop Connection that comes with Vista is not backwards compatible. So if you're running WinVista and your boss or your IT guy is running WinXP, you can't connect.
Fourth: Vista comes out of the box as hands down the single most annoying operating system ever. UAC can be disabled, but wouldn't it be nice if you could get the protection it tries to provide without being bothered every single time you try to run software? It's especially annoying because UAC doesn't even recognize Microsoft software. You can't open msconfig without being asked if you really want to run the program. Same goes for adding user accounts to the PC or changing user account names or passwords. And it's not like you get prompted once, as with Linux. You get prompted EVERY SINGLE TIME!!! You wouldn't beleive how frustrating this is unless you've been there. Also, Windows Defender is some of the worst protective software I've ever seen. It doesn't actually do anything except use up your memory.
Fifth: Vista is the worst memory wastrel I have ever seen. Most of the mid-grade computers these days (what most consumers end up buying, unless they're rich or have special need of a high-end PC) come with 1 GB of RAM. Take, for instance, my wife's Gateway. Windows Sidebar has a nifty little gadget to monitor your CPU and memory usage. When her computer turns on (and I've tweaked the hell out of this, by the way, to keep startup processes to a bare minimum), her memory usage will jump to 98%, sometimes all the way to 100%. This is running nothing but Windows Vista, Trend Micro Antivirus, and Webroot Spysweeper. I understand SpySweeper has a tendency to slow down boot time, but even when I turn that off, it's still really bad. This all means that WinVista is basically forcing itself to use the page file and drag your computer down during startup. After things are started and the computer settles down, her memory usage hovers anywhere between 40% and 70%. Turning off the Aero GUI and nothing happens Try running MS Office 2K7 on top of this, and you're right back up at 80%. It's such a terrible waste.
So WinVista is out of the question for me, even though I'm running a 2 GB machine with a GeForce 7600 GS graphics card. I don't like the idea of my OS using up any of the VRAM that I would normally be using for gaming. There will have to be some serious updates done to Vista before I'll consider installing on my PC.
This leaves WinXP. It rarely crashes on me. I know my way around it well. It plays all my video games. It runs all my software. All that software that you get for free with Linux? It's there for XP, too in most cases. The GUI is simple and doesn't waste memory. All my home-brewed software works fine.
XP, all the way.