Top 10 in my book and yes, I still use ALL of them 'cept IBM DOS to this day.
1. Mac OSX
2. MorphOS (that may surpass OSX for me now that it runs on MacMini)
3. AmigaOS 3.9
3. AmigaOS 3.1
4. Mac OS9
5. AmigaOS 1.3
6. Atari ST TOS 1.04 and beyond
7. Commodore 64 Basic/JiffyDOS/GEOS
8. AtariDOS 2.5
9. TRS-DOS
10. IBM DOS/MSDOS