Which operating system is "better", in the sense of "more useful" depends entirely upon the person who is going to be using it. To drastically over-generalize, for gamers, I'd recommend Windows; for casual users, OS X; and for geeks, GNU/Linux or even one of the *BSDs. I'm a horrendous computer geek, so of course I use GNU/Linux - Gentoo x86-64 unstable, to be exact.

However, this "use what's right for you" is a cop-out of a sort. Just because how useful an operating system is depends on who's using it doesn't mean that we can't objectively compare them in some fashions.

To choose just one possible criterion: if you want to know which operating system is best-designed (from a programming/engineering standpoint) of those commonly used, I'd have to say GNU/Linux, because it's extremely componentized (so much so that what exactly constitutes the "operating system" is rather debatable) and open-source. Windows doesn't stand a chance against it, and the only thing OS X has on it is the fact that a lot of it is written in Objective-C instead of C, and that it uses a microkernel (sort of).

I prefer Linux. Mainly because Vista is just so over the top. I haven't had much trouble at all with Vista in terms of crashes and such but I've always liked a simpler interface. The integration of the terminal into even mainly GUI based distros is a plus as well. There are so many things that are just easier to do with a keyboard than with a mouse, especially on a laptop.

OSX

Linux is better overall.

For "newbies" or "non-technical users". Gnome and KDE are both easier to use than Windows. Linux is also cheaper than OSX, which has a similar difficulty than Gnome and KDE. So, Linux is better.

For geeks... Linux is better.

For gamers... Linux runs PS3, and since PS3 is the best gaming platform, then, it is obvious that Linux is better.

For mobile users, Linux can run any mobile device, while the same cannot be true in Windows or OSX.

Linux, in fact, can and does operate such devices as microwave ovens and satellites. So versatile! Linux rules.

Unless you meant, which OS is better for the PC. In which case, Linux consumes less resources, is cheaper, is super easy to use, and can run most computer programs designed for Windows (using Wine or VirtualBox). Ergo, Linux rules.

For me it's Ubuntu (or Kubuntu). I get my work done, it supports all my hardware far better than Windows ever did, there's loads of software, it's safe(r) and it's free. And yes, I can still play "Team Fortress 2" and "Warcraft" :)

I remember this question from way back when I was a hardcore Amiga user arguing with those silly Mac guys :)

Ubuntu. It can be installed on most systems, it's free, it's open source , it's secure, it's loaded with useful software, it's lightweight, it's updated with a major release every 6 months, it's free and it's free. Why should you need to buy a new PC to keep up with your operating system?

Windows XP. Everyone knows how to use it already and is used to it. It runs all the latest games, has tons of programs for it, and is good for all kinds of office use because everyone knows how to get it to work.
Linux you have to think too much, Apple you have too much eye candy not enough business tools, and Vista is slow. Dell keeps selling XP because people want it. At the moment, it's got the comfort of something you know and trust, and if it doesn't, it doesn't take a special genius to fix it.