I shouldn't, I really shouldn't - but I'm going to anyway...
One could use this argument to justify that Justin Bieber is better than Skrillex, and I think I know where we all stand on that one...
There are a number of issues that come along with having the same piece of software on everything. The biggest one is security. Imagine you are a malicious hacker, and you know of an expliot in the operating system that everyone uses exclusively on all of their machines. All of the sudden, you can expliot every machine. For a user, this is bad. Now imagine that Everyone uses an operating system that is customized to their liking on their machine, and that they chose the kernel, that gui, the network software, the startup daemon manager, all for themselves, and that there were many options out there, and it was unlikely that more than 10% of machines had a similar configuration to the one that you have found a vulnerability on (let us not mention the config files themselves...). You are royally f***ed, and should consider another profession. For a user, this is good. The first scenario is basically what Windows has going on now (especially what with their backwards compatibility to the DOS days). The second scenario is what Linux would be if it were the primary OS of the world, because there are so many options as to how you can set up the OS.
As to why Windows is on all of this hardware, despite the obvious drawbacks to such a scheme, lemme tell you an evil little secret. Windows gets slower as it updates. This is intentional. As your machine gets slower and slower, you are more likely to go out and buy a new machine with new and faster hardware, and ultimately, a new OS if it is available. But wait, what if you took the bloated pile of Winblows off of your machine, and replaced it with the latest version of Linux? All of the sudden, it runs as fast or faster than the day you got it - because the machine was never really slow. Because Windows intentionally gets slower as it updates, hardware vendors really like supporting this OS, because it also encourages users to buy more of their (newer, and potentially more expensive) hardware.
Enough about Windows, now it is OSX's turn! If Apple were to have their way, they would have all of the security problems that Windows has, but worse. This is because OSX has a very limited pool from which people can choose their software. But I think you get the idea now, so let's talk about stability! Apple, because of it's user model, requires that all of the official and officially approved software work reliably, without bugs or security vulnerabilities (not that they don't creep in anyways). Because of this, the software is a good 2 years (or more...) older than what everybody else is using. You wanna know why there aren't nearly as many good games for OSX as there are for Windows, despite the development process being far more unix-like (and therefore friendlier to the developer)? Here's a hint: The latest version of OpenGL that OSX supports is 3.2. On every other OS, it is 4.3, and 4.4 for NVidia users willing to run beta drivers. The 3.2 specification was released in 2009, 4.2 in 2011, 4.3 in 2012, and 4.4 about 2 months ago. And what was it about OSX being more stable? Oh, wait - they must be talking about this: https://discussions.apple.com/community/mac_os/os_x_mountain_lion
Ooh, my favorite one is probably this one: https://discussions.apple.com/message/22935836#22935836
EDIT: Actually, this one is definitely more amusing than the 'kernel panic' one: https://discussions.apple.com/message/22936875#22936875
EDIT: I epecially like the bit about the "Spinning Beach Ball Of Death (SBBOD)"
It should be obvious by now that I voted for Linux. I have been running Linux for over 2 years now. I ran Ubuntu for a year, then CentOS for about 2 weeks (that was a mistake...) and then I switched to Arch Linux and haven't looked back.
Now, even though I still maintain that Linux is vastly superior to Windows or OSX, I have to admit that it has its own issues. The package manager thing still has a few kinks in it, and the idiots at RHEL need to get the idea out of their heads that Sun's solaris is an OS worth imitating. But all in all, it has worked far cleaner and reliably for me that Windows ever did. I can't imagine that I would ever have been able to learn so much about computers and technology without it. As it is, I am currently working on a real-time hardware-accelerated beam-tracing rendering engine that is based on OpenGL 4.3. It is in pure C, and I am using Clang as my compiler.
There is a project that I would love to see become a success though. It is called haiku. It is a reimplementation of the BeOS specification, and it looks awesome. The package manager is still under development, but looks to be written such that it will have none of the problems that the other package managers out there have. When this becomes a workable general-purpose OS with the same kind of utilities as the common Linux distros, I am planning on trying it out.
I had way too much fun with this...