Windows XP is the current holder of the title of the Best Operating System in the World as of when Windows Vista was released. This is because it lacked a few features that made Vista bad: Parental Controls and User Account Controls. This is good because people do not have to be restricted from doing things that they want to do. Also, Windows XP has Microsoft Sam, a Text-to-Speech Voice for the Text-to-Speech converter that is built in.
Windows 2000: [link]
Windows Vista: [link]