The comments here have left me daunted.
I'm guessing there's a bit of trolling going on, but wow, just wow if there isn't.
Now, I'm not an expert in security, but I'm pretty sure the basics aren't that hard or complicated for anyone here to grasp. The pedantics of which operating system is inherently more secure are too difficult to derive, as no body knows how many undiscovered vulnerabilities remain in each OS or how many will be created in the future.
If a particular OS has patched 400 vulnerabilities last year and another only patches 50 vulnerabilities, would you say the one with 50 patches is more secure? How could you know if that OS only has 50 vulnerabilities as opposed to 50,000?
Saying that my OS is better because you don't run as admin or my UAC prevents vulnerabilities or my market share is too small is naive at best. An exploit is code that uses vulnerabilities to execute code irregardless of your security design. If even 1 unpatched exploit exists for your operating system, you cannot claim your OS is secure.
The real measure of the security of an operating system is by the patch response times to vulnerabilities found and active wild exploits. The longer you're forced to remain unpatched to known vulnerabilities, the more insecure you are.
I haven't seen a vulnerability patch response time report in a few years, but the last one I came across showed ~ 4 day response time average for Linux kernel patches, ~ 45 day response time average from Microsoft, and ~ 4 month response time from Apple. (This is from memory, my apologies to fanboys if I'm off a little)
The only factor outside this that I can think of is the fact that Linux is open source and if so inclined, one could create their own patches much easier than with the other operating systems.