MS Windows. It's the biggest and worst piece of I deal with everyday for the last 20+ years. It has cost me and other colleagues more hours/days of productive time than any other tool ever imagined. I can use/maintain/hack UNIX/Linux, Mac and Windows OSes but I truely regret having to spend any time on a Windows machine. For those of you who haven't tried a Mac, you owe it to yourself to check them out. Macs are to Windows, Glocks are to black powder pistols.