For Mac users, it's generally between Firefox and Safari. Safari is faster and renders pages more accurately; Firefox has more extensions and plugins. I favour the latter, and the Glims plugin for Safari adds all the additional functionality I need. The only uses I have for Firefox are a) to test what web pages look like in Firefox; b) to spoof http headers so I can view America-only video content in England.
I've played around with the IE9 beta and it looks very promising. I'm thinking it will be my browser of choice on Windows. Anything less than version 9 is pretty awful compared to the competitors, but MS is slowly catching up. Betas aside, the usual choice for Windows users is Chrome vs Firefox, with the same set of advantages and disadvantages as in my first paragraph, with Chrome standing in for Safari.
True enough, but for me, when I used FireFox, I ran a lot of plug-ins, but I find that I only use one extension with Chrome. I suppose I could, and probably will look for more sometime, but I just don't tend to think about it...