Worst OS : Windows Me (understood) Worst non-Microsoft OS : MacOSX Best OS : Windows XP Professional edition followed by windows vista. In my opinion windows vista ultimate is better than XP but i am simply not used to it. But i have not experienced any serious bug in Windows Vista. It is definitely more secure, while XP has been a target for viruses for a long time and they manage to find a vulnerability in it because of the enormous number of people using it. Well can't be helped, some people definitely found the experience with Windows Me really good, but the reason remains unknown to me. And I have noticed that most people who are against windows vista have not tried it, but only one of whom I know managed to find much faults with Vista. There has been an enormous fixes in vista and is reliable.