There is NO “best” operating system, that’s why we have several major ones. If you point out that Linux dominates servers and the Internet, the fact remains that Windows continues to dominate the desktop with Mac OS X a distant second and Linux making nary a ripple. There is no more best OS than there is a best motor vehicle. Some do better with a dump truck, others with a sports car with a manual transmission.
Poorly written software for Linux can have memory leaks. There is a lot of poorly written software for Linux. We have to reboot Linux environments at my workplace regularly, not as often as Windows Servers, but far from bullet-proof. Same goes for Mac OS X, on which my six year old was able to perform hard freeze-ups just with Safari and LEGO.COM. (Solution: ban LEGO.COM, but it should be impossible to kill the machine from the browser.)
A LOT of hardware does not work with popular Linux distros. I doubt I would get full use out of my Fujitsu ScanSnap s300.
A LOT of software for Linux is second-tier. Libre Office word processing is not as featured as Microsoft Word, let alone Corel WordPerfect. There is no Linux substitute for FileMaker Pro. A lot of the best tools for Linux have been happily ported to Windows without loss of functionality. There is no useable Linux equivalent of Adobe InDesign.
Microsoft makes a LOT of developer tools, and many programmers find them superior for their work to offerings in the Linux world.
For home users, while the OS is a significant portion of the cost of of a store-bought computer system, it amortizes very nicely. Windows XP Pro bought in 2002 was good for over ten years. I have gotten six solid years out of Windows 7.
Points about security are well-taken, and if Microsoft ever tries to force an Adobe-style subscription model on the customers, there will be an opportunity for Apple and the Linux distro-makers to seize.
The best OS is the one that works for you.