I actually use 3 browsers.
Primary: Chrome
By default Chrome seems to be the fastest for me. The browser is extremely lightweight, and gives more visual space for pages.
Secondary: Internet Explorer 8
Since Chrome isn't compatible with everything and every site, IE8 is usually a sure bet. At work, I use IE8 for work related browsing, and Chrome for my personal stuff.
Finally, Mozilla Firefox comes in when there is some weird compatibility problem in IE8, or when I want to use the 'Download Flash Video' add on. I really like that add-on in FireFox, to download any youtube videos!