Windows is the de-facto standard OS. Over time, it has been enhanced to the point where it's stable and it contains practically all the features one might want. Its main advantages and disadvantages relate to its popularity. It has the greatest selection of software and support for a very wide variety of hardware, but it also has the greatest variety of malicious and deceptive software that users have to watch out for.
Mac OS X is pretty and it provides a pleasant user experience. However, it has various limitations, some of which can force one to use Windows for some tasks. It's also tied to expensive Apple computers, and it has worse hardware support. It's a good choice for those who don't do stuff that runs into the limitations and don't mind paying extra for computers.
Linux is free and there is a large selection of free software that can run on it. Some of that software is incomplete, buggy and/or ugly, but there is high quality free software. Linux has some limitations due to not being the de-facto standard, but the situation is better than with Mac OS. The main disadvantage is that a lot of GUI software is kind of awkward and not pretty, as if it was designed by programmers rather than skilled user interface designers and graphic designers.