IMO, "fast" or "fastest" are highly relative terms. In my experience Opera is _the_ fastest, but this is after it's loaded! Ditto for Firefox. FF without extensions would be just as fast but again the question of usability remains. Chrome is the fastest to load, so essentially you get to work before the others have loaded. Add those extensions and it's the same as FF.
On Ubuntu FF is the default browser so it has the advantage of a fast base system. But Opera on Ubuntu is again faster than FF.
Opera has several modes to help you browse even faster. And the built-in email client has a 'low-bandwidth' mode which no other email client has. Overall, Opera is way ahead of the others.
However, if you do a quick search on this topic you'll know that the browsers handle javascript differently, so depending on what you're currently doing the speed will vary from one site to another. So no clear winners.
If you're too lazy to do your own homework (esp. the guy who started this discussion) the one short answer: Chrome.