I have various opinions on this. First I will say that I am a Microsoft Certifed Professional, however, I am also a ardent support of Apple. Now, having said that, lets get to the nitty gritty
I feel that the best browser right now that is Google Chrome. I am blown away at the size of the browser, the speed that is obtained, the structure of the browser and the design. It keeps a very small footprint on system resources and is very secure. Having said that if you need active-x support with a company web site, or other sites, then you are stuck with Internet Explorer. I use IE for accessing my customers Remote Web sites, and Chrome for all of my personal browsing.
As for my Mac's, I am torn. I want to use Safari. I really do. As a strong supporter of Apple, I love to use Apple's products, especially on a Mac. That being said, up until the most recent release of Safari, I found myself using Firefox on my Mac's. Like I said, till the most recent update of Safari

Safari 4.0 offers some serious stability improvements as well as some very nifty new features. Check it out if you are a Mac user and have been using other browsers.