Have you used those OSs? It really depends on what your criteria are. If you are talking about usability today or about historical significance. CP/M has tremendous historic significance but not much usability in 2010...
10)
9)
7)
6) AmigaOS 3.9
5) BeOS/Haiku
4) Windows XP
3) Linux w/KDE Window Manager (a "good distro" with package management)
2) Windows 7
1) MacOS