Firefox for sure. With Firefox's plugins it easily haves more features than any other browser plus it does not have any of the flaws or security vulnerabilities of IE. Firefox is also nicely cross-platform.
I sometimes use Konqueror. It is nice to use while going through files as it is also a file browser. Lastly, on the rare occasion when I need a browser on the command line I will use Lynx.
I would never use IE as it does not render pages that well; I remember the browser Olympics putting Opera and Firefox tied at first with IE dead last. IE also haves the massive gapping security whole produced from its odd use of active x and similar languages. IE is also very slow to catch up with technologies, as it just got tabbed browsing. Something Netscape, Firefox, Konqueror, and Opera haves had for, at least, a little while.