Well I've had my share of experiences in the land of Operating Systems:
Worst:
DOS - My hatred for this OS dates back to my teenage years, it sucked PERIOD!
WinNT - like Wacoon said, it was not as stable as people made it out to be.
Win98 - it was more buggy than Win95!
WinXP - though probably not as bad as previous versions Windows, it's still quirky, clunky and unpredictable.
Mac OS X (10.0) - Though I like Unix based OSes, this one was just plain SLOW!
Best:
VMS - probably one of the best networking type OS out there. It was very stable as well, it's a shame that DEC got bought out by a half-baked PC company called Compaq :-(
Solaris - This is probably the next best OS behind VMS with the same +s. I've worked around this OS for nearly a decade now :-D
BeOS - Just a cool all-around OS that would practically turn any slow PC/Mac into a fast performing machine. I still use this OS now.
Linux (the RedHat distro is my most favorite) - Though a few years ago I probably wouldn't have listed this OS, I quite like it these days. A very good and cheap alternative to the more expensive/proprietary Unix Oses out there.
Mac OS X (10.1 and up) - though I was not exactly happy with the intitial release of OS X, I have been a satisfied user since the release of 10.1 and I am looking forward to Panther's release soon.
AOS - what's not to like? :-P :-D ;-)
