In no particular order.
Favourites
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MacOS X - It just does it's job so well without getting in the way of actual apps, and combined with iApps and Apple hardware it's the first TRUE home computer since Commodore bit the dust. It doesn't look too shabby
AmigaOS3.x - Just for simplicity and shear tinkerability
+3DOS - Simple, easy nice with a clear text menu interface
RiscOS 3 - Very nice at the time, was perfect for the education market it was successful in. My first experience of a GUI, and my introduction to digital art which later lead me to the Amiga.
MS-DOS - I used to enjoy being geeky and making a boot disk for every single game I bought. :-D
Windows 2000 - In my stint as an admin I had to look after lot of Win2000 machines, and I found it a good stable powerful and relatively smooth OS. The only real issue I ever had was with a driver for a USB-Serial adapter on a laptop (Which wouldn't of been an issue if the dumbass network engineer who went out and bought laptop would have bought one with a serial port like everyone else had - especially as half it's use was to console into routers and switches via serial :-o)
Worst
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MacOS 9.x - I can't believe people prefer this over OS X, it has to be the worst, most unstable, overly cluttered, ugliest OS ever made. I tried it once on my dual boot eMac and when installing a program is crashed halfway through and killed the system, making it refuse to boot.
Windows 95 - I couldn't believe how bad it was when I was forced to downgrade from DOS/Win3.11.
Windows XP - This is a odd OS, if you run it and have no problems it's a pretty good windows version, but if problems hit, they hit BIG time! It's telling when the UK's biggest PC reseller has to attach a recovery information leaflet to every recipt of every computer they sell.
Amiga(D)OS 1.x - UGH! It's no wonder Amigas ended up becoming a toy/game machine when they were supplied with a toy GUI like this one had. No standards in the UI (even different gadget syles in the OS itself!!!!)
