Opera. The speediest one I've ever seen, great features, great customization options.
Firefox is probably its biggest competitor but they are lacking in speed. With the all the extensions that duplicate Opera's functionalities it becomes sluggish. Moreover, there is not a built-in feature to export those extensions.
Opera's minuses: some sites loaded with ajax, java, etc do not run properly on it; the built-in RSS reader and bittorrent client are too basic to be useful (the built-in IRC client is not bad though).