Opera is very fast, seemingly the fastest browser I've used on a Mac. But I just couldn't stand the look of its toolbars and icons. (A matter of personal taste, not a real criticism.) So I switched to iCab -- which is similarly ugly, except it's not too bad when you use text-only toolbars and change the toolbar color to graphite.
(Yes, I realize you can do text-only toolbars in Opera -- but they're ugly too. Ugh.)
iCab isn't quite as fast as Opera, but it seems as fast as MSIE, at least. And it has a feature I totally dig, the happy face/unhappy face that you can click on to see HTML warnings and errors.
Another thing I like about iCab is that you can control what things scripts are allowed to do. I'm not allowing scripts to open new windows, take away my toolbars, move or change the size of my browser window, access referers or my history list, or write in the status bar.
However, my favorite browser remains OmniWeb.