Well, I'd have to say that AmigaOS is probably the best OS. EVAR. True pre-emptive multitasking years before any other commercial OS (yes, even Mac) had it, all in 1MB of RAM.
However, it died due to lack of support, which brings up a slightly more important point:
The best OS is not the OS that is best from a technical standpoint. It is the OS that lets you do the things you want to do, and run the software you want to run.
That being said, any Win2k or XP are really great. Linux is great at being a server, though I do know a few people (mostly zealots) that try and use it as a main desktop OS.
I really don't get Macs any more. PCs have caught up to them in graphics ability, and almost any decent Mac software is also available on the PC at this point.
BTW, I know a few people that use Opera and seem to really like it (on the Win platform). Haven't had a chance to mess with it too much, but the gestures rock
