It's popular
The best thing about Windows is that it's popular. This means there is lots of software for it, and nearly every device you can find has Windows drivers. Which in turn, makes Windows more popular.
It isn't necessarily the best operating system - actually, I like Ubuntu better - but there are some advantages to using what most other people use.
On the other hand, popularity is a curse, too. Not many people bother writing viruses for Mac or Linux, but Windows is a big fat target. It's nice to load Ubuntu and not have to run an antivirus program. Those things are almost as bad as viruses themselves.
The ideal situation, I think, would be for Windows, Mac, and Linux to have about equal market share. Then all of them would have a good selection of software and drivers, and they'd have to compete on other features.