It really depends on what you are looking for in a browser. Internet Explorer has been around for ages, and has the full support of Microsoft behind it. It is sleek and powerful, but on old(ish) computers that may slow you down a bit. Still, as the standard browser, you won't find many other negatives with it.
Internet Explorer 8 Beta 2: http://www.microsoft.com/windows/products/winfamily/ie/ie8/getitnow.mspx
Mozilla FireFox is a highly customizable browser. New themes and gadgets are released every week, and this gives a personal feel to it when viewing websites. Although it slows the computer down, it isn't as bad as IE, though on a fast computer, you won't notice much of a difference between the two.
FireFox 3.0.1: http://www.mozilla.com/en-US/firefox/all.html
Other options available are Opera Browser. This browser offers very limited customization features (only a colour change in fact), but because of this gives very fast browsing. it doesn't require as many resources as IE or FF, but it isn't very fun to browse in something that looks as out-of-date as it does.
Opera 9.52: http://www.opera.com/download/
The final major browser is Apple Safari. In my opinion it is the fastest of the 4 browser and my personal favourite. Its style is based on the iTunes layout and it doesn't offer any customizability, but if you like iTunes, you'll love this browser. Its major downfall is the fact that it doesn't render (load) webpages very well. This can become very irritating while browsing so most people don't give it a second thought. It does run very fast (even on slow computers) and has the fastest startup time of all browsers. If you find that the pages you usually visit work fine on Safari, you won't have anything bad to say about it.
Safari 3.1.2: http://www.apple.com/support/downloads/safari312forwindows.html
A new introduction to the Browser market is Google Chrome, Google's first attempt at an internet browser. It is currently in a beta version, but it seems to be one fo the fastest browser out there. But, as it is still in beta, there are many stibilty and security problems with it. I would recommend against using it as your primary browser, for the moment at least, until the final release comes out.
Google Chrome 0.2.149 Beta: http://www.google.com/chrome/index.html?hl=en&brand=CHMG&utm_source=en-hpp&utm_medium=hpp&utm_campaign=en
So it really depends on what kind of person you are and what type of computer you have. You can't go wrong with IE or FF, but under stress, they perform slowly, Opera is a boring looking browser but can run where the other two can't, Safari has problems with rendering many webpages and there are far too many security risks with Chrome, but is much the fastest of them all. I personally use all five of them (but IE and Safari are my main two). If Apple ever get the rendering process sorted, it would be my number 1 choice.
Hope that helps.