It depends what you want to do on your computer.
But for general use, in my opinion:
1. Windows XP (best Windows OS ever)
2. Windows 2K (like XP but without the bells & whistles and is getting a bit outdated)
3. Windows 7 (A very goos OS dispite the fact that I do not like the new inefficient taskbar and hate libraries)
4. Windows Vista (lots of different opinions on this one. I do use Vista regularly and I must say it is a decent OS, not as good as the other 3 but still decent)
But there other OS options as well, such as Linux or BSD.