I think most popular and one of the first operating systems is a Windows OS, which also is mine favorite Operating System. Why? First of all, it is really easy to use, even your little brothers and sisters can use it, without any knowledge. Second of all, throughout the time of evolution Windows become more convenient , operating interface is clearer and better with years. There are also other different OS , such as Linux, MacOS, Android which also have their advantages and disadvantages as well.