You should have included a poll in the thread.
For me it is Windows, hands down. Windows is a great operating system, it is flexible, fast, easy to use, and has a great interface. And, because it is the most widely used computer operating system in the world by a large margin, more software is developed for it and is reasonably easy to find solutions for any issues it may have. All software I need run on Windows. Microsoft Office is absolutely great on Windows, and it is the single most important piece of software for me. Most games run on Windows. And Windows run on nearly any computer, so I have choices whenever I want a new one, which adapt to my needs and wants. Windows is not perfect, and it certainly has its issues. However, if I have Windows running, I do not need any other machine.
Second, it is maCOS. It is also a great operating system, has great aesthetics and is easy to use. Macs are usually good computers and integration is great. However, software is somehow lacking. Sure, macOS covers all the basics and then some. But many cross-platform software are conversions from Windows and not all of them have the same quality. And many are not even available. When using macOS, I sometimes feel like a second-class citizen. Microsoft Office runs on macOS, but the Windows version is miles better; this alone is a reason for me to opt for Windows. Plus, macOS only runs on Apple hardware, which means less choices. It is easier to buy a Mac, but also more constrained. And there is timing: it is not wise to buy a Mac shortly before Apple announces a revision of its hardware.
Third, it is Linux. I tried different distos, but it is seriously lacking. While I could handle anything with Windows and macOS, I have a hard time adapting to Linux. It lacks polish; the interface may have become better over the years, but it is still nowhere near Windows or macOS. Plus, software is seriously lacking. First, there is no Microsoft Office for Linux, which is reason enough for me not to use it. But Linux also lacks other software. For office apps, at least there is LibreOffice, which is decent, although nowhere near Microsoft Office. I found that most other open source software lack polish and the quality found in commercial applications which were not available for Linux. I would love to be an enthusiast of open source software, but I simply cannot because the software I tried was usually terrible, with only a few exceptions.