OK, I've kinda skim-read most of the previous posts here. So here's my opinion.
Best OS? OSX. Without a doubt. I don't own a mac, I can't afford one. I've used Windows since the 3.1 days, and Linux since the very early days too. Given a choice, I'd buy a mac. It has the best gui, the best underlying architecture (essentially unix - although you'd never know it) and ... well it just rocks. The whole OS fits together wonderfully, and it's a complete joy to use.
As I can't afford a mac, I use a mixture of Linux and Windows XP. Any machines I want to do serious work on run Linux, and when I just want an easy life I run XP. Sometime's that not correct - as every now and again XP needs to be reinstalled - but generally, it's a good "get home from work, remove brain, read email and surf the interweb like a drooling zombie" OS. It's not the most efficient OS, it's poorly engineered, the GUI is still 10 years out of date (albeit with fischerprice themes to 'pretty it up') ... and well, if you've used many other OS's the shortcoming of XP are obvious.
However, each to their own - whatever OS you have most experience with, is probably the OS you'll consider 'the best'. It's like religion - happiness with the familiarity of something, a belief that your beliefs are the best and everything else is inferior. A bit like HIFI too really
As for browsers - I generally use IE or Konqueror (which I believe is built on Mozilla). I have very few problems with both - although Konqueror does occasionally render 'broken' pages as broken. This is more down to poor site design and MS not sticking with standards - but very few sites seem to suffer from poor code nowadays. I'm reading Zero Gain in Konqueror - not problems at all, and it seems faster than IE. I've not yet tried Firefox, but I've been told it's the dogs danglies.
Steve