I've been a Firefox user since around 2003 (when it was called Firebird) and that's been my web browser of choice. At work I use IE 6-8, and it does the job well too. I've tried both Chrome and Opera, but that was a long time ago so I can't really say how they are today. I've upgraded to Firefox 4 at home now and I think it runs great, I'm very pleased with it so far. I haven't tried IE9 yet but I hear it is great too.
Short answer: Firefox