CP/M-80 on an Amstrad PCW (sadly trashed some time ago
) - simple command prompt system, a joy to use. Failing floppy disk drives and failing 3" floppy disks aside.
Locoscript II on the same machine - this was basically a word processor with integrated OS. Really nice for the time, with functions that are still not offered by modern day word processors (eg. design you own fonts, use a two key combination to copy *and store* blocks of text for later use while copy/paste still worked normally (and you could have up to 10 blocks of text stored!)).
Then came Dos 6.2 and Windows for Workgroups 3.11. That system I loved for its relative stability, and the fact that its internals were quite well documented in files that were included with the OS. Oh, and all in approx. 20 Mb. Add office and a good drawing package and it would still all fit on a 100 mb harddisk with room to spare. With Win32s and WinG you could even run some early Windows 95 programs.
Windows 95 - that was awful. Gah. Bloated in comparison to 3.11. Not so stable.
Windows 98SE - probably my second favorite windows. Very stable as a gaming system on my old PII. Lacks the internal documentation of 3.11, and the interface has the same problems as Win95...
Windows XP - HATE. Takes the Win95 interface a step further, adds loads of really badly documented features (unless you look on the internet, and know where to look on MS's horrendous website, I suppose), frankly it just lacks something. Bloated.
Vista - I think I hate this one just from what I've heard and read about it.
BeOS - would still like to try that one.
Linux - Too badly documented for beginners IMHO. Now the last time I tried it was a few years ago. I suspect it has improved.
MacOS 7.x-8.x - Tried this at Uni a long time ago, kinda sucky (machines were quite old/worn), but easier to use than Win95/98.
MacOS 9.x - Better, tried it still at uni a long time ago. newer machines too. Liky. Reason for buying a Mac after my AMD-powered PC decided that snacking on harddisks was a good idea.
MacOS 9.2.2 - Quite fast, quite stable, a nice workhorse. Interface decent, but not very flashy.
MacOS X 10.2.x - Better than 9.2.2, a lot more stable, nice eye candy.
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MacOS X 10.4.x - Fast, really really stable, a joy to use. I still prefer the way 10.2.x looks, though.