1. Windows XP SP3 (x64 Pro) - To be honest, most stable OS i've ever come across, took M$ long enough.
2. AmigaOS 3.1 - Uttery stable, whenever it isnt, it's usually hardware (or software faults, not OS).
3. AmigaOS 1.3 - Got to love them colours!
3. Linux (Mint & Knoppix) - Mint for it's nice layout, Knoppix for it's design and cleverness.
4. Windows 7 - Already a hell of a nice OS.
5. Mac OS9.2 - New user of this but wow, is it stable and easy to use!
6. RISC OS - Takes alot of effort to bugger this OS up lol.
7. Morph OS - Very clean.
8. Atari ST TOS 2.04 - Same comment as RISC OS.
9. Spectrum Basic - Simplicity is bliss.
10. AROS - Just wish I could afford it.
P.s. Great topic idea.