For which OS?
Please see my statement on portability. For a web browser -- or, indeed, any program -- to be considered truly great, it should not matter which OS we are talking about.
Firefox is the only major web browser that has both:
a) been ported to at least four major platforms, and
b) been released as free software, so that it may easily be ported to any new platform.
Chromium is catching up, but it is
not quite there yet.
Thus, Firefox is undisputably the best general-purpose web browser of all those with significant market share, and possibly overall. Please note that while some other web browser may be better suited to your own purposes, this does not make it any better a
general-purpose web browser.