Firefox, both for the sweet, sweet Adblock and the impregnable fortress that is Firefox's security. Once you get comfortable with that, you may decide to get other addons to totally customise your browsing experience.

I have no experience with Opera, so I can't comment, but Chrome is like a streamlined, fast Firefox, without the Adbloc, so that's my secondary browser (Incognito is useful as well....) You should definitely dump Internet Explorer though. You'll be continually asking yourself why you didn't do it earlier.