I'm under GNU/Linux - thus, it's down to Firefox, Opera or Konqueror (I sometimes use Links).
With the Web Developer extention, Firefox is the ultimate web dev machine: Javascript errors logging, source code highlighting, generated content, CSS error reporting, direct on-page CSS editing...
Opera is nice and fast, but its using of both W3C and IE DOMs makes it unsuited for cross-browser development - so I develop under Firefox, cross-check with Opera and Konqueror, then fire the virtual macine to repair the damage under IE6 and 7.
IE7 is abysmal in its lack of innovation: it is nothing more than a bug fix and code cleanup of IE6's engine, with an horrendously frozen interface glued on - in my eyes, it's a mere beta of a bug fix version that should have been out in 2002.
Let's not mention that it's buggy as hell, and that there's NO way to report said bugs to the devs (Mozilla, KDE and Opera all have some -fast- return on bug reports).
If you want a FAST browser though, the late alphas of Firefox 3 leave every and all browsers in the dust: the new rendering engine (Gecko 1.9) and graphics interface layer (Cairo) make it blindingly fast - while improving CSS compatibility even more.