No doubt Microsoft windows is the “most used” operating system in personal computers, including laptops and netbooks. Not “most popular”, rather “most used”, as popularity comes with freedom of choice. Nearly all windows users are using windows just because it came preinstalled in their computers. Same argument applies to Apple and OS X. On the other hand, Linux users have to pick Linux just because they think it gets the job done for them well. In my experience, most Linux users are the ones who had previously tried different operating systems and had to finally stick with Linux. This is not to imply that Linux is the best operating system, but rather to say it has certain unique advantages compared to other operating systems. It is free, as in freedom, and again generally free, as in cost. It tends to be reliable and has multiple distributions, which gives you the choice to pick the one which suits you well. Techgradar has a nice article on this, by Nick Bothwick.