The most popular browser among the 'regulars' here in AB's Technology section is almost certainly Firefox:
http://www.mozilla.com/en-US/firefox/new/
It's the simplest to customise (by, for example, installing a British English dictionary for spell-checking, or by using Adblock Plus to make this page load far more quickly).
Chrome is also popular (and it's quickly developing a range of add-ons similar to those for Firefox).
Internet Explorer is generally despised and derided. (There have been lots of reports lately of IE9 almost grinding to a halt when trying to load Answerbank pages).
For the fastest loading, try K-Meleon:
http://kmeleon.sourceforge.net/
As Chuck say, you can have as many browsers as you like on your computer. Try them out to see what suits you best.
Chris